Personally I would vote for South lounge, and stay there until about 45 minutes until departure (maybe a little more time if C gate). Your aircraft could leave from many parts of T5, as already noted, and given that uncertainty then South lounge has some more food options than North or B. Bacon rolls until 09:30, porridge until 12 midday (though there's also porridge in Manchester). If you end up at C gate then you can also have £15 worth of Starbucks merchandise from there too.

The one exception would be in the unlikely event that South was crowded, and your flight is shown as B, and there don't appear to be too many other big aircraft at B. In that case the B lounge can be a very calm place to be.

Food menu in post 1 of this thread, photos further down.
Galleries Club lounge LHR: Dining menu and food options 2013