Originally Posted by pythonisman

It's easiest to enjoy the T3 lounges if your next onward flight departs from T3. Most (possibly all at the moment?) BA flights to the US from LHR depart from T5 so on that sector you'd have to use a more complex method to get to T3 for the lounges, then out of T3 and over to T5 for your departure.

The BA lounges in T5 aren't a great place to spend 6-7hrs (similar to above, I wouldn't personally want to transit for 6-8hrs in a lounge, have done it a couple of times due to delays and it's not the most fun.

Instead, would you consider leaving LHR and heading somewhere in London for the time? The Elizabeth line isn't too pricey to get in to Paddington (or onward once you get there), and there is plenty to see for a few hours in that part of central London. If I were faced with a 7hr layover at LHR I would strongly encourage getting out of the airport, get some air/daylight and stretch legs before a next onward flight. (Obviously everyone's circumstances are different and you'd have to consider what to do with hand baggage etc, so this is thinking out loud only!)