The official way is to go to the customer service desk by Gate 11 and request to leave the premises. They will then find someone to escort you out, but it may take a period of time before someone is available (30 minutes I've heard of). There used to be an hourly, on the hour, smokers conga, but someone pointed out that it was somewhat iffy to "encourage" smoking, so that was the excuse they needed to stop that service.

The unofficial way is to take the transit to C (not B), ignore the recorded instructions to get off the transit, then it will take you back to the UK Border. If there is a HAL staff member working the area - unlucky if that happens - they may want you to leave, but if you stand your ground and say you wish to go to the UK Border I doubt they could do much about it. If you used this route you would then need to get a reissued boarding pass in Departures should you wish to go back in, and if you will not be going back you would be well advised to tell BA that you have offloaded yourself.