They simply can't. A UK visa won't let you leave the UK. Same issue for the hundreds of thousands of foreigners on visa or residence permit here. A residence permit issued by a Schengen country also serves as a Schengen visa. I know a lot of Russians that live in Germany and would like to visit the UK but can't without going back to Russia and applying for a UK visa.
Tell your MP that the UK should join Schengen, would make life a lot more convenient for airplane owners
