Generally, new and longer-distance trains will, older suburban ones won't. Quite a few have them in first class, but not in standard.
CrossCountry Voyager trains have power available in most seats, and a nice seating plan:
East Midland have power on their 'Meridian' trains but not their HSTs - link
South West Trains don't generally - it appears that some of their carriages do (the non-first-class driving cars of the class 444 units at least), but it's not publicised...
Virgin West Coast
Super Voyager have power at all window seats.
Pendolino First Class have power at all window seats.
Pendolino Standard Class have power at window seats with tables.
Virgin East Coast
Seats with tables have power (all first class seats have tables) link