As long as it's that way round, you'll be OK on the Priority Pass front. There are Priority Pass lounges in T1 and T3, but not T4 or T5. There will be a third party lounge in T2 when that opens, but it hasn't been confirmed yet if they'll take Priority Pass or not.

In T3, there are two lounges available to you, the Servisair one and the No.1 Traveller. I'd strongly suggest the latter, No.1 lounges are generally some of the best third party lounges out there, should be miles better than Servisair.

With a 9.5 hour layover, you would have time to see a bit of London, or something nearby (eg Windsor), assuming you get let in. Plenty of threads on here with ideas if you do think you might be allowed (eg under the Transit Without Visa landside concession or something). Otherwise, you may well find that 7-8 hours in the lounge (taking account of change-of-terminal times, security clearance times, getting to plane etc) is rather long...