Arrived from Europe last year (FAA ASEL/ASES). Not sure if the CAA require a mountain rating, but if you're planning much mountain flying the best advice I got was from my Dutch BFR instructor and that is to perfect your stall turns. It's all too easy to fly into the wrong moutain valley and find it suddenly narrows or ends and is covered by cloud.
A proper stall turn is a real lifesaver!