I've only had experience with Newcastle and Heathrow. Can't say much for Newcastle as it was a domestic flight in and everything was easy-peasy, the whole experience was quick and simple.
I've officially labelled Heathrow as a pain in the arse. Though I must say that I don't think much of many airports around the world as I'm too spoilt by my own.
Let's take my experience last month for example. I had a AMS-LHR-NCL flight. I flew into Terminal 5 and didn't have an immigration card as I wasn't given one on board. The staff member at the airport who was supposed to be giving info didn't know where to get them, and there was a whole bunch of us on connecting flights to Newcastle, Glasgow and Edinburgh (bunch of Scots in kilts were in Holland for the World Cup qualifier) in 45min. I had to
1) queue to have my boarding pass checked
2) queue to have my boarding pass scanned
3) queue to have my passport stamped and to be "interrogated" by the very stone-faced customs officer
4) queue to have my photo taken (and while queueing I saw another queue forming to go up the escalator which nearly gave me a heart attack)
5) queue for a very very long time in a very very messy hall to have my hand baggage scanned. After which I had to run to my gate
6) queue to board the flight. Had to go through one station where my boarding pass was checked and photo-matched, then another one metre away where my boarding pass was scanned.
Why can't things be streamlined? It was also the same when I departed from Heathrow, there were two separate scanners for bags and shoes which I did not think saved time at all. And I was surprised how small Terminal 3 was for an international terminal, there certainly weren't enough seats for people around and many were lying on the floor.