As I said in my above post, it's not possible to go airside in T5 without a T5-departing boarding pass. As the lounges are airside, you can't access the T5 lounges.

You could try your luck getting into the Japan Airlines business class lounge (before 12:00) or the Servisair lounge (after 12:00) both in T3, which BA are using as temporary lounges - although I doubt you will be allowed access.