Ambassador, British Airways; FlyerTalk Posting Legend
Join Date: Apr 2012
Location: Leeds, UK
Programs: BA GGL/CCR, GfL, HH Diamond
Posts: 42,964
on the way out as your LHR departing flight is from T3 you will have no access to the airside departure area and lounges at T5. You can use any of the F lounges in T3 (BA/CX/AA) or the QF lounge - personally I would use the CX F lounge. You will need to show your inbound F bp as well so keep hold of that.
on the way after arriving at T3 you use the airside transfer buses to T5. once through security you can use the lounges in T5 including the CCR
have a look at the T5 connection guide (see my signature) which may help you