I think it depends where you arrive and where you depart.

Personally, I don’t usually find T5 to T5 too bad, although I agree that this “conformance” thing of BA is a stupid idea and should be switched off. I am continually astonished that LHR has almost no clean arrivals (although my understanding was that domestic arrivals should be clean, even when departing LHR from a different terminal- maybe it has changed…)

Connections between other terminals at LHR are often pretty painful and involve awkward bus trips through tunnels and alleyways- to be avoided where possible IMHO.

FRA is, famously, a complex maze… It’s partly because of the multiple separations of pax for which the airport simply wasn’t built- Schengen/Non-Schengen, clean/non-clean. Also, the UK abandoned physical passport checks for departing pax many years ago, whereas strangely they still have to happen in nearly every other country. On the other hand, the signs are much clearer than they used to be and generally quite accurate, so following them should at least allow you to make your connection rather than missing it.

In both cases, it helps to know your route in advance.