One engine that is not in TSW2020 is the BR Class 55 Deltic with its unique opposed piston prime movers (there are two per unit, by the way). I like these engines, as well as other interesting and rare engines (like Fairbanks-Morse, Alco, and Baldwin diesels in the US) Several Deltics have been preserved and are in operation. This engine would be perfect for an East Coast Main Line Route or for running excursions on existing British routes.
Mmm... Deltics... Hope I'll be forgiven for linking this: Have to take issue there I'm afraid- opposed piston engines weren't unique to the Deltics even in Britain. There was the Baby Deltic (classified Class 23 although I don't know if any ever carried TOPS numbers) which had a single engine with nine cylinders in three banks of three- basically half a Deltic engine hence the name of course. They were more trouble than a toilet full of snakes, all went to scrap and weren't missed by anyone.
Errr let me think for a time messured in micro seconds!! Yes please! Would love the sound of Napiers if only on postal services on NTP!
55's have ventured into Paddington at least once. As for 23's, none ever recieved TOPS numbers, and although troublesome, they were missed by a few, so much so that there is a attempt being made to build one at present. Also, Deltics have appeared on WSR as have 45's.