The only flyable Spitfires available at the time the movie was made, were 'low back' MkXVIs, and a 'high back MkXVI (not used due to continuity considerations) - MkIs and MkIIs (not MkIII) were as rare as hen's teeth, with only two or three in museums at that time. The aircraft used (taken out of RAF storage) were rather worn, and had code letters applied for the movie, which made the general appearance look even worse, more so as filming progressed.
Many of the Hurricane sequences were taken from the earlier movie 'Angels One Five', and the other ground shots utilised MkII Hurricanes, again with faded paint work, more likely the later grey/green scheme. One of these was an aircraft now with the BBMF, which, at that time, hadn't been formed.
The aircraft were taken out of RAF storage, given a quick service, and used, very briefly, in flying sequences, which were 'cut' with actual wartime footage, and models.
The single AVRO 504 was re-engined, and taken from the Shuttleworth Collection (where it still flies today), whilst the sole remaining Bulldog again came from RAF storage, and now resides in the RAFM, Hendon. Some of the Bulldog scenes used a very accurate, purpose-built replica, which was deliberately broken for the crash sequence, hence the reason the Bulldogs and '504 looked virtually pristine.
Filming was done at Kenley, which 'doubled' for at least three airfields as the story unfolded. Bader himself was not impressed with the production (or the final film), and proved difficult, if not impossible, to work with as an advisor. It was not until the early 198o's that he eventually, in hindsight, admitted that the movie was pretty good !
It might seem strange today, with such a large 'warbird' community, and accurate colour schemes etc, that the movie used somewhat 'shabby' aircraft, but it should be remembered that the war had only been over for ten years, and was still painfully fresh in the memories of most in Britain. Britain was still recovering, and cash wasn't available for the massive expenditure made on movies today (the total cost of Reach for the Sky was in the region of £350,000, a huge amount back in the early 1950s!), therefore such things as converting aircraft to the desired appearance, re-painting etc (as was done for the BoB movie in 1968) was really not an option at the time.