As a GCH you could also use the AA Arrivals Lounge at T3. From looking at aa.com it does seems to be open:
https://www.aa.com/i18n/travel-info/...als-lounge.jsp
Tube or Heathrow Express are the recommended landside transfer options. Not sure which would be better though as it's been a while since I have transferred other than via the airside bus.