On the outbound you will have ample time to spend some of it in the lounge - T5>T3 isn't the easiest connection ever but nor is it the worst, probably would budget around 45 minutes for the connection (it's possible to do it in less), leaving you with a good hour to enjoy the lounge. You will not have access to the T5 lounges without a flight departing from T5 therefore you'll have to use the T3 lounges on the outbound flight (of which there are several very good oneworld choices). The widely acknowledged best lounge is the CX F lounge and given you'll have access I'd recommend heading straight there. If you're interested in trying them all you could also take a look into the QF lounge (which is very new) and BA's Galleries First lounge (although personally I'd skip it entirely, nothing special).
On the return flight you'll be slightly tighter for time but should still get chance to glance into BA's Concorde Room at T5 briefly (again you won't be able to use the T3 lounges on the return flight, only T5), which is where you should head if you have an F flight departing from T5.
Enjoy!