I don't think this has anything to do with countries (security isn't immigration) and just with 'extreme caution' and perhaps a lack of trust in their own airport settings. The way it is set up it's basically impossible to get from one part of LHR to another without clearing security.
I've cleared security multiple times at LHR within a short period of time because I had to go to a transfer counter in one terminal and then return back to another terminal. On that particular trip I cleared security four times in a 2 hour span due to IRROPS. I must have been the most secure man in the world.