There are First Class Lounges at various London rail terminals - Euston
(Virgin), St Pancras (Midland Mainline), Kings Cross (GNER), Liverpool St
(One), Paddington (First Great Western) and Waterloo Intl (Eurostar).

Which do you like/dislike? I love the Eurostar lounge at Waterloo Intl
(wonder what the St Pancras one will be like) and Queen Victoria's waiting
room at Paddington. The One lounge at Liverpool St is a bit minimalistic,
though.