I would just fly in - choose your airport wisely - take your chances with the immigration officer and hope for the best. I would dress up, and also have a printed copy of your return flights. It is a risk, but in the absence of the correct visa, it always would have been - but you knew that I'm sure. Good luck.
The UK is a shit show at the moment but it still has a lot to commend it if one is British. And so I'll continue to spend many months of each year here.