I think BA will be quite strict on this because if the onward flight were to be cancelled they wouldn’t have a ticket leaving the UK so BA will always want to ensure you have the right to entry all the countries on your itinerary.

The chief immigration officer could amid them temporarily until they fly onward, they could be derailed at Heathrow, unlikely I’d hope if elderly until departure or returned to point of origin at BAs cost with a fine which is what BA don’t want.