Someone's connection (they on BT retail), has packetloss on TCP traffic to certian destinations, but not on udp or icmp. Which to me I can only think of one rationale explanation which would be some kind of traffic management dropping the packets. Of course BT retail no longer has traffic management on their unlimited packages (at least officially) so I am trying to find another reason. Of course also not ruling out shaping on the other isp's involved in the transit.
The packet loss seems to occur on all eu destinations that go over third party transit but any over direct peering are fine.