Assuming you have a good couple of hours between your flights, I’d recommend popping into the MH Golden Lounge (upstairs before the aerotrain when you arrive). I believe as an arriving BA First passenger (but certainly if you’re OneWorld Emerald) you’ll have access to the Platinum lounge therein – the showers and a la carte dining in there are rather good (whereas the showers in the J section were a bit grim last time I looked). Last time I took checked baggage, I had time for a quick shower and a bite to eat before taking the train over to immigration and getting to the carousel just as my suitcase arrived (granted that was off an A380, so may be quicker from other aircraft – I think I spent about 20 minutes in the lounge). The aerotrain is pretty quick – about two minutes; I quite often pop over to the satellite terminal to use the Platinum lounge even if my flight is leaving from the other terminal – it’s really no big deal to get between the two. That said, the Regional lounge is quite nice and if for any reason you can’t get into the Platinum lounge, the showers in Regional are somewhat nicer than those in the main (J) part of the main (satellite) Golden lounge (confusingly, the ‘main’ terminal is actually the satellite building…). You’ll be able to use the First/Business immigration lane if you do have to retrieve and recheck your luggage, so that should be pretty quick – KLIA is a relatively small and easy-to-get-around airport, so unless you’re on a really tight connection, you should have a relaxing transfer.
Talking of which – they tend to board the 737 flights at the last minute in my experience; I once arrived late from London for a transfer to Singapore, and ran across to the gate in panic-mode expecting to miss the flight or at least be the last one on, charged past what I assumed were the passengers for the next flight using that gate, got ushered down the jetbridge at about two minutes before scheduled departure… only to discover I was the first one on! So of course I then had to sit there while the aforementioned passengers boarded behind me… Slightly embarrassing 😉
Oh and in case it’s useful to know, security / x-ray is at the gate as well as immediately after immigration.