IINM, LHR was (and is still) an issue of slot restrictions and what airlines were allowed to own those slots, not anything to do with US-UK flights. ie, China-US flights are restricted because there is no open skies treaty. PMCO could fly to the UK (and did, to LGW), but couldn't fly to LHR basically because it's too busy.
No, PMCO for most of its history was excluded from LHR due to route authorities, not slots, under
Bermuda II. They
also did not have any slots, because they couldn't use them even if they did.