Yes sharing rooms the way Americans do is unheard of in the UK. Combined with being legal to drink at 18, adequate public transportation (in most places) and less financial dependence on the parents, it means the students lead a far more independent lifestyle.
I might have mentioned this before, but at my Oxford college in the early 90s there was always a few pounds to be made at the end of the academic year helping the porters move extra beds into all of the rooms. The visiting American summer study/tour abroad schemes would of course have their guests share.