With BA, you would have been allowed off the aircraft as long as the jetty is still attached or the stairs still fitted and the aircraft door open. If you have hold-baggage, we may try to discourage you to avoid delaying the other pax further! (.......well, we might try to discourage you anyway, but at the end of the day, it's your decision!!!)

I don't know the full legal ins and outs, but in the USA the airline is legally bound to return to stand to let you off the aircraft until it's actually rolling along the runway!

..............bit unfair on the other poor saps on the aircraft who get delayed though!!!