You say you have a long layover - be aware that all of the lounges at LHR get very busy, and most of them enforce a three hour limit - especially at busy periods. This is particularly true of the Centenarian Lounge who, in my experience, won't allow entry to anyone until three hours before departure.

Premium passengers don't get any lounge entry by default.

The Clubhouse doesn't impose time limits, and each UC passenger can guest one other passenger in provided they are on the same flight.

It is also possible to pay for a guest (subject to space availability) - IIRC it is £80 per adult, but that is from memory.

Nick