You can check British timetables on www.nationalrail.co.uk. The global pass is valid. But the fare isn't very high so if you have a flexipass, you can save a travel day and buy a normal ticket.

Where you come from does not matter at all.

FROM LHR there are 2 trains: a fancy express direct to Paddington station-there have been many reports that other as indicated this will NOT accept passes-its price is skyhigh, as fits most airport links.

AND the re is Hr CONNECT=makes morestops, use this and change to another local train when it joins the main line westbound. If you plan to use trains adapt that you will have to fetch for yourself and its not USA-style waypointers at every stage.

Thank you for your replies.  This was very helpful for two seniors.