I do this quite frequently in T5, was there a week ago and there was a HUGE queue for transfers, about 300m long. I went through the UK border and back in through North Security in about 15 mins, according to others the connections queue took closer to 90 mins!
(edit) And I disagree with drspear, even going through the border and back is often still quicker - even with fast track. The 90 mins I'm talking about here was for FT. Think that was just a bad day, though.