I would allow 3 hours
I've just endured the most incompetent and frustrating experience at LHR on what should have been an easy transfer from T5 to T3. First off, our plane was 30 mins late due to the permanent holding pattern at LHR. Then:
1. Plane to T5 lands and parks near T3. Then we ar on buses to T5. This takes 20 mins while we load the bus and drive through the tunnel
2. We then wait for a bus to take us back to T3 - exactly the same route - we are by now 30 minutes behind where we should have been
3. Immigration. Not too bad by Heathrow standards, but fast track not working - 20 minutes. This is pretty good for LHR. Budget an hour.
4. Security. Looks good, short queue. Unfortunately, my bag, that had already been screened in Edinburgh was pulled aside for inspection. It was behind 3 others. This is where the blood pressure really starts to rise. There is one operative inspecting the bags. She moves with the pace of a tortoise and proceeds to get the first bag. She then slowly removes every single thing in the bag. It's a big bag, there's lots of stuff. Books are ruffled through, every electrical item is swabbed with a piece of cloth, this takes an eternity. She shuffles across to the explosives detector and puts all the electrical items back through the scanner. She brings everything back and then offers to repack. The bag owner says yes. This has taken 10 minutes. The same process on the next bag. The owner of the bag in front of me is going absolutely mental. He's late for his flight. He's had a few too many. His voice is raised, he's arguing, he's cursing security. We've now got a stand off, the operator is preferring to argue than inspect. Security get involved. Suffice to say it took just under an hour for me to get through. The reason for getting pulled was that I had 4 fish oil capsules that had fallen out of a bag and were rolling around the bottom of my bag.
So, my 3 hour time budget and anticipated sojourn at the T3 Flounge and spa turned into a horrible and frustrating journey through the purgatory that BA and LHR security seem uniquely capable of delivering.
Rant over