Different rules and regulations apply within the countries of registry. However as your flight was to the UK, all airlines should comply with the UK's legally mandated direction regarding who is ordinarily permitted to be on the flight deck. Failure to comply is a violation of the requirements for a companies permission to operate, and non-compliance runs the risk of sanctions including ultimately the withdrawal of permission to operate the service.
There are people who may not be in uniform and who may otherwise be permitted to remain on the flight deck. This isn't necessarily obvious to the casual observer, and care should be taken in those circumstances not to allow that presence to cause concern to anybody else on board.
There has been no change to the UK governments legally mandated instruction to airlines, and passengers are not ordinarily permitted on the flight deck during flight. This is mandated for all UK airlines and for all foreign airlines whilst operating in UK airspace.