I have a 10 hr stopover in LHR. Would I be able to get in to the lounge (Aspire) with priority pass on arrival? Is it accessible from arrivals? Would they let me in 10 hours before the flight?
It is hard to know what or what not they will due since by official policy they let you in for 3 hrs ("Access is permitted 3 hours prior to scheduled flight departure") although I have had varied experiences with lounges and since there is more than one priority pass lounge you can use one for a few hours and then move on to a different one in many cases.
In regards to arrival why would you not continue to check-in for your continuation flight immediately?
p.s. note the Aspire lounge is scheduled for temporary closure: "THE LOUNGE WILL TEMPORARILY CLOSE FOR REFURBISHMENT WORKS BETWEEN 03APR - 30SEP25"