Originally Posted by
BertieBadger
Exactly correct.
The "Ready to Fly" marker becomes a problem if it thinks you've already cleared security and are airside. So simply exit at Heathrow.
As an aside, many BA golds flying onto other UK airports do exactly this, because while they don't plan to leave the terminal, they can re-enter via The First Wing which is generally more pleasant than connections.
It's not helpful to potentialy confuse everything and talk about Golds or First Wing when the OP is not, even as an aside.