Priority Pass listing suggests that the Air Canada LHR3 Arrivals Lounge is closing:
Hence the past tense, closure to coincide with AC moving to Queen's Terminal, have sadly made my last visit to LHR arrival, many twenty minute showers there (there are typically more showers than clientele, indeed many times there were more personnel than clientele. Not sure what new arrangements are to be made for LHR Arrivals with terminal changes, it has long been part of the few distinguishing features of LHR that airlines competed on arrival services rather than only departure.