Yes you can use the pass everywhere else for the full 3 month validity but must pay separately for any trips to/from/within UK.
As GB only has this €* as train on rails to get out/in: it will only take passes valid for the whole route, there is no chance at paying separate for 1 country-so this means, if you insist on trains on rails, to buy a normal full fare. The good news is that f.e. for late oct/early NOW these are quite cheap-only a few gbp more as the 30 extra to pay on a pass.
As you likely are very much aware too: you could also fly out/in or use the bus (coach for GB).
Or check seat61.com for other ALT by ferry etc.