Tom, I think EU 261 will only apply to EU irlines. As Britain will leave in 2 years time it would be a logical step for BA, Easyjet and other British airlines to scrap this rule. It would save them a lot of money. I believe that under a Tory government the UK will revert to a US-like situation where the level os service and consumer protection is absolutely sub-standard compared to the current EU situation.
From a passenger’s point of view, especially those living in the EU, it would be a reason not to fly a British based/registered airline any longer.