Assuming this is seperate tickets then you'll need to clear immigration and collect your bags, which means you can't use the transit channel. Immigration can be horribly slow at times
But once you get to domestic check in then most Malaysian tickets allow you to "go show" for an earlier flight for no charge so see how you are on time
The Domestic lounge is rather lacking in food options and no booze. Not sure on showers. Unless something has changed you'd be OK to access the satellite lounge immediately after you disembark the BA flight though before you take the train/immigration.
#ftr it's actually not too bad getting over to KLIA2, the trains are frequent - RM2 but can pay with credit card. Not much in it if you have to clear main immigration anyway. Different if you are able to through-ticket or go HBO and stay in the transit channels