How many squadrons of flying boats and shore based aircraft had been flying anti-sub patrols in 1917-18?
It is worth noting the almost universal ignorance that everyone has for the use of aerial maritime patrol assets in the Great War, notably airships, which were quite effective in that role. By the end of the Great War the RAF, formerly RNAS had over 100 maritme patrol airships at air stations dotted up and down the United Kingdom. The non-rigid airship made an excellent maritime patrol asset for several reasons, they could carry a sizeable load across a great distance and had excellent endurance, they had sets of eyes in numbers and could be deployed from any sizeable field with personnel for mooring; aside from the main airship stations, there were satellite mooring-out stations to assist long patrols when crews couldn't make it home.
As for their effectivity, the use of maritime patrol is as much a deterrent to U-boat attack as much as brining about the destruction of enemy submarines. At the end of the war (WW1) it was officially claimed that no enemy attack was successful against vessels escorted by airships, not strictly true, as two freighters were torpedoed whilst under escort from airships near Cornwall, but that is a pretty good record considering that from 1915 to 1916 the RNAS began building up its maritime patrol airship fleet. The Germans dithering over whether they would launch a sustained campaign meant that U-boat successes until 1917 were not large by comparison to that year, which meant British maritime patrol assets were not as numerous in the first few years.
From 1917 and the declaration of unrestricted submarine warfare, sinkings trebled. By the end of 1917 a greater tonnage of Allied merchant ships had been sunk in the previous 12 months than during the previous three years of the war. This caused a boosting of funding and concentration of effort to produce anti submarine assets, including airships, - nearly 200 non-rigid airships served with the RNAS/RAF during the war. Oh yes, and submarines were sunk by airships during WW1. The U-boat UB 115 was sunk by the rigid airship R.29 with help from the destroyer HMS Ouse (which was the pattern of aerial attack to call in surface help once a submarine was spotted) in September 1918.
U-boats tended not to attack if there was an airship present and post-war research (carried out by a chappie I once knew called Patrick Abbott, who wrote an excellent little book on Britain's use of airships during the Great War) showed that German U-boat commanders typically abandoned attacking ships because aircraft posed a serious threat to their survival. Once they were spotted, they would dive, but because their sub-surface speed was low, they could be caught by surface vessels relatively easily. Best to remain invisible and slink away without making an attack, rather than make an attack and be caught and sunk.
It is interesting to note that the United States was the only country to use maritime patrol airships during WW2, retiring the last, in training use in 1961. As for the use of hydrogen, the British only lost a couple in lifting gas related accidents, employing strict handling procedures, which the Germans, who lost quite a few airships due to accidents with lifting gas, did not employ.
What did the RAF require on the eve of WW2? More of everything, basically. More aircraft with greater range and better sensors. I (vaguely) remember reading something that was said post war by the then head of CC stating that specifically two things that helped win the Battle of the Atlantic was the VLR Liberator and the cavity magnetron. Now, if only I can remember who said it.