I’ll just reiterate using the connections route won’t get you through the UK border to enable you to exit landside - which is what you want.

So there is no point in buying a BA ticket that you won’t use as a way around that because you’ll still be in the airside connections stream to take you to T5 and no easy and quick way to get landside.

Yes when flying say FRA-LHR-EDI you do clear the UK Border during connections but that doesn’t get you access to landside (where you want to be) at LHR.