The USAAF did fly Spitfires during the war. I think the reason so many Mustangs survive is that they come from much later in the war and with the dawning of the jet age many were either sold to other countries, private individuals for air racing or moth balled "just in case" whereas at the end of the war Britain was destitute and anything no longer serviceable for a combat role was melted down to make stuff. Tragic but true... hence why so few Spits and the like. There are only 2 flying Lancasters left, now that's rare! I believe a Blenheim is currently under restoration that'll make it the only one flying and I also know about a Ju-88 restoration. Slowly some non-flyers are being returned to the air for our joy.
The lancaster is another plane I am dying to see fly. I have the A2A Lancaster from Wings of Power in FS2004, but nothing replaces the real thing

You Brits used your Spits and Mosquitoes until the mid-late '50s albeit limited service. With a decent amount of countries having used Spits I expect more to be flying. I do have to say though that there is group producing replica fighters at the moment. Usually (maybe all) are slightly scaled down (~3/4) but aerodynamically and structurally they are very close to the real deal:
http://www.waraircraftreplicas.com/I have seen a ~3/4 scale Spit at Oshkosh many times, beautiful aircraft, but I have not even seen that replica fly.