Just check your bags in at the HK Island station at IFC or Kowloon. You can check it in in the morning - in fact anytime that day up to about 2 hours before departure. You will need your Octopus card, it will be scanned before you enter the baggage check area, but you can still travel on the Express in the afternoon. In both stations BA's counters, shared with Finnair and others, is on the left hand side.

Having done that, you can enter the lounges at any stage convenient to you.