I recently spent some time at the BA LHR T5 Galleries First and I was quite impressed. I'm pretty sure it was the South lounge (next to the F check-in's and security). The lounge is huge, loads of space, a la carte menu, food buffet, plenty of wine/alcohol choices, a business centre, comfy seating choices and even a terrace overlooking the runway and the inside of the terminal. [Can't comment on their Business Lounge though as I've never been].
QF J Lounge at MEL and SIN are both very good J lounges, when comparing with other J lounges. But then again, theoretically (and it happens imo) is that the home carrier lounges at their home airports should be the best lounge at off all the carriers eg. QF is the best lounges at SYD,MEL; SQ at SIN, BA at LHR, EK at DXB and so on, etc.
IME QF J at SIN is probably equally as good as SQ KF J Lounge. QF F Lounge at SIN is almost as good as the SQ KF F Lounge, but where SQ trumps QF at SIN (and this is expected given the home ground advantage) is the SQ The Private Lounge.
However just a thought that when flying with QF/OW to FCO it requires three flights if going via LHR, whereas SQ will only require two. (SQ I think flies the 772's from SIN-FCO though).