Hi PZU - a T5 to T5 transit is as painless an experience as you can get anywhere. I see no difference whatsoever between LHR T5 and those lauded world airports such as SIN, HKG and so on. It is just the British mentality that we don't like to celebrate what we've got
Actually I would put T5 ahead of HKG (which had very poor transit security arrangements when I used it) and it is streets ahead of AMS.
I say this as a former LHR hater who came to loathe the T4 to T1 flight connections horror etc. So I empathise. I should give it a try, it is definitely a credible transit option once again.