... nested quotes trimmed ...
Doesn't account for the poor BQM (assuming his using IPv4 for that)
True, but congestion at any part of the sky network could cause that and you can't diagnose that without traceroutes.
I know what your saying but it isn't congestion in my humble opinion, If I was to guess I would say its an obscure issue with the Openreach bit, but we all have very little to go on.
I basically don't want to see the OP charged. Two OR visits are now saying the line is ok, and that may be true at the time they were there. what he need to do was show a slow download after the OR download attempt.
I keep on coming back to this laptop, we know that Gbe nics are common place and been around for a good 20 years on pcs. the op needs to use a spare known good router and a known good pc to test on his line.
I don't understand the issue around his laptop because the information is soo sparse. If he's brother is on the same pon and thus same olt port at the exchange then he will have same issues and that can be raised in tandom to his own line issue.
Until we see more information, the issue remains at his laptop.