Someone may be able to confirm or otherwise, but certain routes require lighter locos than Class 66s (due to weight limits on bridges, gauging issues etc.) - so you'll always get the smaller/lighter 73s on them. Also, if it's a route predominantly on the Third Rail, it makes sense to use 73s which can run on electric on the Third Rail rather than diesel.
As for Class 20s, as noted above the only regular workings these days are of the tube S Stock transfers from Derby via Banbury to West Ruislip - GBRf operated usually with 2x 20s at each end. You'll also sometimes find the 20s in West Ruislip Underground depot between workings.
If you search under RTT for "West Ruislip Lul Dept GBRf" (e.g.
http://www.realtimetrains.co.uk/sea...18/02/17/0000-2359?stp=WVS&show=all&order=wtt ) you'll see the paths in for the Banbury-West Ruislip moves - however it's one of those that's there's lots of paths but only certain ones get used for delivery runs. I'm no expert on when they happen, so you'd need to hunt round the forum a bit to try and find out when they're running - alternatively keep an eye on the Derby (Chaddesden Sidings usually I think)-Banbury workings as I'm guessing if something moves from Derby to Banbury, it's logically then going to move to West Ruislip. e.g. Derby-Banbury 7X08 working last week...
https://flic.kr/p/EB8wyK
