I can't get my head around the fact an abundance of Spitfires were sitting around RAF airfields not really achieving much while the Americans were so desperate for escorts they were bolting ferry tanks onto P47's which were unsuitable to the task so they at least had something.
An over simplification. They were not just sitting around, not achieving much. An examination of the Fighter Command, Group and Squadron records would reveal the activities being performed by all those Spitfire Squadrons that were supposedly "sitting around".
First task, and the highest priority they had allocated was defence. Protect the airfields, the supply chains, factories, all the facilities within the UK that needed to be defended against aerial attack to allow the conduct of the war and operations from the UK. Protect the ports where the supplies were coming in from overseas, protect the convoys on their final stages of their travel into ports and out of them; protect where the US forces were arriving into the UK, protect the areas where the troops were concentrated and training and preparing for the invasion.
Then, within the capabilities and range of the various marks of the Spitfire between 1941 to mid-1944, conduct offensive operations or operate in support of offensive operations against targets in occupied Europe. Early on that entailed escorting the like of Blenheims, then Stirlings, then more so the light/medium bomber aircraft of 2 Group (Bostons, Venturas, Mitchells, Mosquitoes) on bombing raids against targets in the Netherlands, Belgium and France. As things progressed, those operations were often against targets such as airfields and conducted to coincide with the ingress or egress of USAAF heavy bombers against targets elsewhere. On occasions for target within their range, they did provide the escort for the USAAF heavies when operating against targets such as the French ports, U-Boat pens in France, etc - especially when the weather forecast for targets inside Germany were unsuitable. Later still, they did a lot of ingress and exit escort for USAAF heavies on return from targets in Germany. And large scale fighter sweeps in support of the operations of the 'heavies' - tying Luftwaffe units to protecting particular areas in the Netherlands, Belgium and France or disrupting their efforts to intercept the 'heavies' when called in as reinforcements.
Escort for Coastal Command operations against enemy coastal shipping along the coastline from up near Texel in the Netherlands out into the Bay of Biscay off the French coast.
On top of that they still continued their programs of Rangers, Rhubarbs and other offensive operations against targets within their range, attacking flak posts, railway engines and rolling stock, Luftwaffe airfields, Army camps, supply and fuel depots, early morning and evening sorties trying to catch E-Boats and German minelayers and mine sweepers operating in the Channel and North Sea.
Then if you look at the records, when the USAAF medium bombers, primarily the B-26 Marauders started operating from the UK, a high percentage of their escort was provided by RAF Fighter Command units, leaving the USAAF fighter units to concentrate on the long range escort role.
Into late 1943 and early 1944, the Spitfire units, particularly those on the Spitfire V, get their bomb racks and start performing fighter bomber operations against initially transportation targets, bridges, then become very heavily involved along with the Hawker Typhoon fighter bomber units, in the attacks against the V weapon sites being constructed in northern France. Another aspect of that was the campaign operating as fighter bombers against the railway marshalling yards and holding yards where the V weapons were coming in to supply the launch sites after they were built.
Add to that the training required to improve unit efficiency, armament practice camps, train with Army units in the CAS role, periods of rest and maintenance after long periods on operations including integrating and training up replacement aircrew (often with a secondary operational role protecting rear areas even whilst notionally 'on rest').
And I'll also throw in there, the number of the RAF's Spitfire Squadrons from mid-1942 onwards who were hosting USAAF pilots for periods of operational experience - usually somewhere between 2 to 4 weeks with a Squadron. Often this was in the timeframe between when the units the pilots belonged to initially arrived into the UK and before the aircraft they were to operate arrived by sea from the USA and were reassembled and made ready for operational use. A number of the USAAF fighter pilots who were to fly with the 8th and 9th Air Forces got their first operational experience in the ETO in a Spitfire belonging to a RAF Squadron as a part of that program. Was helpful in getting them used to European weather and the way the airspace in and around the UK was controlled and some exposure to the theatre of operations with experienced RAF pilots.