Yes, CO passengers have access to the Air Canada Arrivals lounge at Heathrow (I believe that it is open to 1.30 or 2pm, anyway). Have used it once or twice (am normally on the LA flight - one of which arrives too late to use it ). The showers are really nice (clean, stylish, nicer a lot than the CO showers at IAH) and there is plenty of space to relax in and a reasonable buffet breakfast and selection of drinks. I liked it - definitely better than the CO Arrivals at LGW (where the showers are nice, but the rest of it is small and like a dentists waiting room...).

For the Air Canada Arrivals Lounge, walk outside of the terminal you arrive at at Heathrow (Terminal 3) near the bus stops and it is along a bit on the left. It is signposted from within the terminal and, hopefully, the Co Concierge will give you an invitation and map to get there (although you don't need an invitation to get in, just your boarding pass).

What with access to the Clubhouse at EWR, you should have a great trip

Boo