Go to any BA desk, even the lounge entry agents can offload you (provided they’re not busy with a queue).

Officially they may ask you to wait for an escort which is typically every 30 mins to an hour to escort you back out through the border. The quicker unofficial way is to go to C gates, remain on the train and exit via the border that way.

Any taxes or fare that you may be entitled to (depending on fare conditions) will have to be obtained by calling BA later on after the offloading process.