Perhaps my language was a little strong. Going thru immigration and customs might not complicate things, but at the wrong time of day you could end up in long lines to get your boarding pass and clear security. You might end up in long lines at immigration. Last year I spent an hour in line at immigration (busy Saturday morning) without a Fast Track pass. I was staying for a couple of days, but I would have hated wasting that hour on a turnaround run if it meant I didn't have time to shower and eat and rest in the FL/AC.

I really wanted to emphasize that clearing immigration is not necessary if you are simply turning around. An earlier post mentioned clearing immigration and customs without mentioning that those are optional steps. Sorry for the overly cautious tone of that point.

Let's say this: Go thru immigration if you really want to or if you have a reason to. Like you do.

Since there is very little to do at Heathrow outside the secure area, if I have just three hours I would stay inside security, take a leisurely shower at the Flagship Lounge/Admirals Club, eat some breakfast at the FL, have some beverages, watch the early AM news, read the paper, window shop at the T3 departure lounges and then it's time to re-board.

Of course, five hours is enough time to get to Paddington on the Heathrow Express if you want to.