12hrs at ANY airport lounge has to be hell. I'd catch the HEX and go into London for all but the last two hours.
If you intend to enjoy yourself for 12hrs in the Clubhouse you'll realise after a few hours you'll be running out of things to do fast and at that time it'll be far too late to go into London.
You could even hire a car for the day and explore some English countryside, your time is best spent away from LHR!