Well, I was brought up to revere all things Great Western and remember seeing King George V at the buffer stops at Paddington when I was small. Three of the class are preserved. Examples of most of the GWR locos are preserved or, remarkably, being reconstructed from scratch or by conversion of other locos.
Other locos from other UK companies are also under construction, such as the P2, the Patriot rebuild and the LNWR "Bloomer".
Although one of my friends is a descendent of George Jackson Churchward, chief superintendant of the GWR, the loco that I would like to see reincarnated is one from the Lancashire and Yorkshire Railway: an Aspinall "High Flyer" or "Zulu". This was the class that was the second Atlantic 4-4-2 design in the UK, and had 7' 3" diameter driving wheels. When running the Liverpool to Manchester expresses they gave rise to many unproven claims of 100 mph running. In view of the fact that they often ran with light loads it is not outside the realms of possibility.

Robin