I arrived at LHR T5 yesterday from Madrid at about 1730.
There were no queues. The e-gates were closed so we needed to go to join the UK/EU queue. I was in business and first off the plane so the queue was minimal.
My PLF was checked and my COVID test was scanned in detail with the officer checking specific details.
So the OP was have been unlucky in arriving at a busy time, though of course the service was unacceptable.
I have already been contacted by phone to check I am self-isolating (which I am and it is sensible for them to check)