There’s no definitive list, but here’s all the lounges I’m aware of:
Avanti
Glasgow Central, Manchester Piccadilly, Liverpool Lime St, Runcorn, Crewe, Birmingham New Street, Birmingham International and London Euston (future lounges to open in Preston, Stockport and Rugby to the best of my knowledge)
LNER
Edinburgh Waverley, Berwick, Newcastle, Darlington, York (opens later this year!), Leeds, Wakefield Westgate, Doncaster and London Kings Cross
GWR
London Paddington and Cardiff Central
ScotRail
Aberdeen only (Glasgow Queen St lounge no longer opening)
Grand Central
Wakefield Kirkgate only
Caledonian Sleeper (Lounges for all guests)
Inverness, Fort William, Dumbarton Central, Stirling, Perth, Dundee and Leuchars (there may be more!)
Hope that helps, and apologies if I've missed any!