The Range

Could you double its size, height and depth.  Also if that is in pure compost you will need to pot it on.  Compost will only feed it for, say, 6 weeks.  Id move it to a double size pot using JI No3.  Put it outside, someware nice and sunny.  Before Winter, November, it will be dormant, best to get it into the ground.  If you can't then double the pot again and use a mix of JI no3 and garden soil.  Pot on again in Feb, this time into garden soil only.  Its a tough bush and once past this immature stage will stand normal UK weather.  But best in the ground or a really big tub.  (Twice the size of a builders bucket).

Ok, but use soil and compost and put it outside, in a sunny spot.