This sounds like two completely seperate intineraries. My question is, will you have your boarding passes for your connecting intinerary before you land at LHR? Or will you have to get them after you land? I would suggest the longest connection you can find (obviously within reason).
From BA's website: "You need to be at your departure gate at least 40 minutes before departure time"
http://www.britishairways.com/travel...r/public/en_gb