I've not flown UK-LHR-Int'l in a few years, but same terminal connections (e.g. GLA-LHR-Europe via T5) definitely had no screening back then.
Connecting between terminals always requires re-screening since all passengers get mixed regardless of origin, but at least T5 didn't require re-screening for same-terminal domestics. (I can't remember if there are domestic flights into any other terminals?)