It has been a while since I abandoned a domestic flight at T5 but I've done it a few times (voluntarily due to excessive flight delay on day trips) and I definitely was not sent to the border. I remember walking down "secret" corridors and ending up in domestic baggage claim and the BA rep had to speak to an official person along the way but definitely no passport control. This was just a handful of people from various flights, not when a whole flight was cancelled.