Yes, perfectly sensible idea, you can go to both A gate GC lounges at North and South and / or T5B too. Just read up on the passageway guide for departing passeengers, and remember NOT to take the transit when leaving T5B to return to T5A but to use the passageway. The guide can be found in the BA Forum Dashboard. The Concorde Room and First Lounge have additional entry requirements beyond a business class ticket.