Assuming you are BA Gold or above, the F Lounge is excellent. The J Lounge is also pretty good.

The No1 Lounges are becoming a bit of a mess in my experience. Despite Amex Platinum and Priority Pass giving access (that is after-all what you are paying for!) there seems to be an exception that you are going to pay additionally about £15 pre-book for 'guaranteed' access.......all a bit of a scam in my view, when you've already paid once for access.