You pretty much lined it out yourself pretty good.
-Use the possibility to skip the line at the security
- Go to the Concorde Room for Dinner(wouldnīt bother with having dinner on board, unless you really want the experience. But the flight is so short, you barely have time to sleep.)
- When arriving go to the Concorde Room in T5(have a shower in a Cabana maybe?) You can transfer airside to T3. You can have Breakfast there as well. Itīs great.
- Go to the CX F-Lounge in T3, they are having amazing food too.
If you didnīt already book the flights, I would recommend you give yourself more time in LHR to switch the Terminal. 1:40 is a pretty tight fit if you want the full First Class Experience at LHR. But thatīs just how I would do it.
Cheers
EDDMflyer