I have a CE flight in a couple of weeks, departing LHR T5 around 8am. I arrive at LHR on a bmi flight at 9pm the previous day. Is there any limit to how early I can go to the lounge before my BA flight?

Assuming I'm allowed to spend the whole night there, is there anywhere in the T5 lounges that anyone would recommend for a few hours' sleep? I could book a hotel room but it seems a waste of money if I can sleep and get some breakfast in the BA lounge.