I've finished the following:
Northern Rail
First TransPennine
Merseyrail
East Coast
Cross Country
East Midland Trains
Virgin Trains
Grand Central
Wrexham & Shropshire
Hull Trains (never been on one of their services tho)
Chiltern Trains
First Capital Connect
First Great Western
Island Line
Northern Ireland Railways
Very close to finishing London Midland (only Walsall > Rugeley outstanding). For ATW I only require things that have reopened since 1985 (Aberdare, Maesteg, Ebbw Vale and Bridgend > Barry).
Admittedly I've been at this for years and had privs when I was a teenager.
Is it possible to do all the Valley Lines in a day ?
Yes. I did it in 1985, and got back to York afterwards. There were 5 fewer lines then (see above, plus Cardiff > Radyr via Ninian Park), but also only hourly services on most lines, formed of lumbering 116s that were slower than today's trains on the gradients. Should be a piece of cake.