I have read that you don not get extra bagage allowance with BA but I do have a 5 hour wait between flights at Heathrow I guess it may be worth the comfort of the lounge.
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Priority Lounge Pass may make more sense for you, 10 visits for US $249, unless you can get your value out of the QP here ????
I don't believe there are any Priority Pass lounge in T5 since the next leg is with BA.