If you've only got hand baggage then they are not likely to offload you until the last minute anyway, and even if they do and you still turn up in time they can always just put you back on the flight.
Just a minor point but on the basis of painful personal experience I wouldn't rely on being able to do that at all LHR terminals and with all airlines at LHR these days. Once the shutters come down at T minus X it seems to be increasingly difficult to get the computer to change it's mind....regardless of you being stood at the gate, with your hadn baggage, with the jetty still on and the aircraft doors still open.