Well, declaring war and starting the movement of forces from the UK to France in less than 24 hours after the declaration is pretty rapid. However, it takes time to move forces. By 27 September 1939, some 152,000 British soldiers had arrived in France but it still took another 6 days before they started to relieve French troops at the front line.
Putting RN capital ships into the Baltic would be a stupid idea. Those ships would have zero friendly air cover and yet would have been under constant threat of air and subsurface attack. While it's highly unlikely that Stukas would sink a battleship, they could do some serious damage to the superstructure and weapons. Smaller vessels would be at greater risk. And for what aim? To lob a few shells on German ports?
As it was, the RAF was already engaged in operations against German ports, the first attacks happening on 3 and 4 September 1939. Now, these weren't very effective for a whole host of reasons...but that's not the point. The RAF was taking direct action dropping bombs on German ports and military shipping. We also need to keep in mind that the legal aspects of aerial bombing were still nascent. Britain, Germany and France were, at this stage, avoiding bombing each others' civilian infrastructure which generated some thorny legal questions about dual-use facilities like ports.
And Britain did come to Poland's aid, declaring war on 3 September and immediately starting offensive action via the RAF and deploying ground forces to France. Britain satisfied the terms of its agreement simply by declaring war on Germany. Note that this was the first instance in British history where it went to war on behalf of another nation, as per a treaty obligation. Britain wasn't directly under threat from Germany and yet it still went to war because Poland was attacked. That's a non-trivial decision, particularly in light of what happened in 1914-1918 which scarred the European generations who experienced it to the horrors of war, prompting the efforts to avoid a repeat of those experiences.
And there's still no solid concept for what Britain could have done to defend Poland's territorial integrity. I'll touch on your comments about the Saar offensive later, which absolutely was a missed opportunity. However, there was simply no way for British forces actively to engage German forces in Poland. It was a non-starter geographically, logistically, temporally, and from a materiel capability perspective.
I never said it excused the sell-out. I was simply offering one perspective for why it happened.
As to avoiding war, my comment above is entirely relevant. The Great War scarred Europe. Both Britain and France were keen to avoid a repeat. Plus, in 1938, Britain was certainly in no position to respond military. It was still playing catch-up trying to update its military, particularly the RAF. Finally, Czechoslovakia presents similar problems to Poland. There was no plan to deploy forces there and it's a tough place to get to from the UK.
Yes, Czechoslovakia was a sell-out but it directly led to the more determined efforts regarding Poland and, ultimately, the declaration of war that resulted.
Again, what would you have Britain, in particular, do? Britain declared war because Poland was invaded. It immediately undertook offensive action and started moving troops to France. What other options are on the table, aside from putting capital ships into the Baltic so they can be target practice for the Luftwaffe and the U-boat fleet?
Now, here's a point where we can both agree. The Saar offensive was a glaring missed opportunity, largely driven by poor intelligence and lack of intestinal fortitude. I have no idea why France didn't press forward with more vigour. Perhaps they were waiting for the BEF to show up in adequate numbers? It also should be borne in mind that the Great War showed it was desperately hard to advance against well-prepared positions. Perhaps the French commanders wanted to conserve their forces rather than throwing casualties against the Siegfried line? Again, poor intelligence probably contributed to some atrocious decision-making.
One note, though, is that France couldn't use all of those 100 divisions in the offensive. They needed a large reserve in case of German counter-attack (again, poor intelligence plays a role). The plan for the Saar offensive called for 40 divisions to participate. However, France didn't even push forward with that scale of force.
I have no solid reasons or rationale for why France didn't press forward. There was a window of about a month where they could take action against a relatively poorly-defended German front line...and they entirely missed the opportunity. I suspect an overly-centralized decision-making process within the French Army, coupled with fears of becoming over-extended, may have driven the decisions...but that's just an opinion and we all know what those are worth.
By mid-October, Poland had surrendered and Germany was able to reinforce its western borders.
Again, that's an over simplification. Yes, there were plenty of "bombphlet" raids later during the Phony War but, as noted above, legal concerns played a part in targeting decisions, as did Roosevelt's advocacy for all belligerents to not attack civilian targets. We had, collectively, not reached the stage of unrestricted warfare that truly only came about in May 1940.
You're also ignoring the action the RAF did take in response to the declaration of war. The AASF deployed to France on 2 September 1939. As noted above, the first bombing raids against German ports took place on 3 and 4 September. The first RAF air losses due to the Luftwaffe took place on 20 September when two Fairey Battles were shot down.
Attacks against German ports and military vessels continued thru the end of 1939. The Battle of Heligoland Bight was a pivotal event that would ultimately drive Bomber Command to night operations due to the vulnerability of bombers during daylight.
It's worth noting that, in 1939, Bomber Command only had 280 bombers in total. That's a pifflingly small force...which again leads back to "what can they do with it"? Given the lack of technology available, Bomber Command wasn't much of a capable force at all. Perhaps more could have been done but, IMHO, there was little opportunity to deliver any decisive effects given the resources available.
Part of the problem here is that we know how the war played out. We know that Germany would continue its expansionist actions. That wasn't clear in 1939. It's perhaps tough for Americans to understand the depth of the scars left by the Great War. Literally every church in every village in Britain still has a memorial to local lads who died in the Great War. It wiped out the finest men of a whole generation. It's easy to point out the lack of enthusiasm for engaging in another war but most people truly didn't want a repeat of 1914-1918...and, in 1939, there was no evidence to suggest that wasn't going to happen.
Perhaps more could/should have been done. However, there were limits to Britain's ability to actively engage Germany on a large scale. France probably has less of an excuse but, then again, France was prepared for a defensive fight and, for whatever reason, clearly had little interest in taking the fight to Germany. Perhaps the politicians hoped for a negotiated settlement rather than further expansion of the war? Given recent experience, that would be naiive but one only has to look at the machinations going on in London in May 1940 about whether to reach a settlement with Berlin. Again, easy to judge in hindsight...perhaps less easy when the future isn't clear.