Priority Pass terms are very specific:
Outside the US, flight tickets must be accompanied by a valid boarding pass for a departing flight, i.e. outbound passengers only.
Strange then that there are two PP lounges at LHR which are explicitly arrivals lounges:
https://www.prioritypass.com/en/loun...a-dcc2db3c85d2
https://www.prioritypass.com/en/loun...a-dcc2db3c85d2
NB: I've used the T2 arrivals lounge several times. Sometimes they ask for a boarding pass and sometimes they don't. When I have been asked for a pass they have always made it clear it should be an arriving pass and not a departing pass...