Thanks for the summary. I know that a bill was recently suggested by a British MP to reinstate the ability of citizens of Commonwealth realms who share HM Queen Elizabeth as monarch and head-of-state to be able to use the UK citizens line (e.g. Canada, Australia, New Zealand, Jamaica, Barbados, etc.). Not sure the bill got off the ground or went anywhere, but I would have voted for that!!
I miss those days pre-EU when we got to use the UK and Commonwealth line, especially at Heathrow when two or three big jumbo jets have just landed from the USA. Although if we are flying Air Canada Signature Class, the priority Customs and Immigration line is a very nice perk and good substitute.