If joining as a Drummer, then Yes they'll teach you.
Most battalions will put you through Army School of Ceremonial where you will be taught to play bugle, PLUS flute (Bb or F) OR Side Drum. Kitchen Sink (Bass Drum, Tenor Drum, Cymbals) will be a local option at Drum Major's dictat.
If joining as a percussionist (the CAMUS you spoke of) I think they might be able to teach you, but you'll probably need a second instrument (other than tuned percussion).
Drummers are trained from scratch (as above) in Inf Bns. You'd have to be an Infman first. Usually Machine Gun Plt. Some Tanky and other units have drummers (particularly Scottish influenced units) but again, it would be ht and miss.
Grade 5 (IIRC) is required for entry in to CAMus, or you can be auditioned... but you'd be expected to show that you'd already learnt some skills, and had the ability to devolop. (ie you can't just front up and act like animal).
Royal Marine Band Service may take you as a Drummer. They have a different organisation, and their Corps' of Drums is part of the Band, rather than part of the Bn. Again there is an audition, over 5 days IIRC. They ask you to play instruments you may not have used before in order to gauge your adaptability too.
The Corps of Drums in an Infantry Battalion are 'real' soldiers. They go on to learn or develop a second skill as a drummer, bugler or fife player. They normally double up as a Support Weapons platoon (Anti Tank, Machine Guns etc).
The Buglers of the Royal Marines are professional full time musicians and merely an offshoot of the Band. Do not confuse them with fighting men.