Let’s have a look at the Fighter Command strength from AIR 22 - 'Air Ministry: Periodical Returns, Intelligence Summaries and Bulletins':
Op.: Aircraft in operational units.
Est.: Establishment strength in operational units.
Serv.: Serviceable aircraft in operational units.
Crews: Aircraft with crews in operational units.
1).Numerical strength of FC goes up in the period 1939-42, stabilizes in 1943 at a high level and starts going down in 1944-45.
2).Regarding the types of aircraft, initially it is the Hawker Hurricane that is the main fighter but from late ’41 the Supermarine Spitfire takes the lead.
3).Standardization doesn’t seem to be high on FC priorities since many different types are used concurrently. This is understandable in the case of two engine aircraft that are used as night-fighters, such as the Beaufighter and Mosquito.
However when it comes to day-fighters we get in May ’41: Spit, Hurricane, Westland Whirlwind and Boulton Paul Defiant.
Things start to change in the second half of the war. In May ’43 we have the Spit, Hawker Typhoon and Whirlwind.
Only in 1945 does standardization win since there are just two types, the Spit and the P-51 Mustang.
4).In 1941-42 it’s hard to justify the large number of Spitfires kept in the UK since the Germans have a very small airforce in Western Europe. Meanwhile the RAF in N.Africa has to use the outdated Hurricane and the American P-40 Warhawk.
5).Despite all the talk about the Spitfire being the best fighter of the war It is interesting to note that even though FC has numerical superiority it is still beaten in the Channel battles of 1941-42 by the Luftwaffe’s Channel-Geschwader JG2 and JG26. The Germans used the Bf-109F and FW-190 that had superior performance compared to the British types. [Sources: ‘The right of the line: the Royal Air Force in the European War, 1939-1945’, p285 and 561 and ‘Jagdwaffe Volume 4, Section 1: Holding the West 1941-1943’]
6).Throughout the war the main problem for FC is that their ‘star’ aircraft lacks the range to take the fight into Germany.
7).Lend Lease aircraft only appear in large numbers in 1945, when the P-51 Mustang makes up 48% of FC in April ’45. Prior to ’45 only the Boston night-fighter is used in small numbers.