I can sort of see where the OP is coming from.

The UK is pretty much alone in the EU in requiring everybody to be rescreened. The EU has common airport security standards, and normally anybody who has cleared security in either the combined EU/Schengen area, or in the USA (which has signed up to the EU standards), does not have to re-clear security when in transit, unless airport lay-out makes this necessary (as was the case at Schiphol until a few weeks ago).

The UK, in typical UK fashion, decided to opt out of the common standards.

Of course even if the UK were to end its opt-out (pigs might fly!) the current arrangements would continue for the foreseeable future, because of the way LHR is laid out. It took years for AMS to complete the redesign that now allows passengers from all "safe" countries to transit without re-clearing security.