Any transfer at LHR is a pain, even when you don't have to transfer between terminals. The British don't trust any other country's security screening; this year transfer security between flights at T3, en route from HEL to DFW, took almost an hour and at T5, en route from MAD to LAX, it took longer than 30 minutes. To add insult to injury, even when the boarding pass has the Fast Track indicator they don't offer it for connecting passengers.