Agreed. FRA is at least serious about keeping clean arrivals clean, and in some limited cases it doesn’t work due to airport logistics. LHR isn’t at all serious about keeping clean arrivals clean, and in limited cases- domestic arrivals at T5 onto a T5 departure- it should work, although it’s clearly not a priority. The tiny staircase at the end of T5 and the domestic connections area were an afterthought and the central idea was always that even domestic arrivals connecting to a T5 departure go out landside, then back upstairs, then back through security. Pity.