For me, 1st year was alright, everything was introductory, challenging in places but there to help you settle in etc etc.
Second year, the modules were mostly boring which was a disappointment and when you're not really 'enganged' with the stuff, it's not fun. Then, for some reason, it looks interesting again in the third year.
In terms of difficulty I found that yes, it does step up slightly but not kind of hugely? I'd say that it's kind of similar to (and smaller than) the gap between Year 13 and first year of uni.
But also don't forget that second year is probably not the end of the world if you don't do particularly well - third year probably weighs more. Also don't forget to get involved (in uni and outside of uni too), look for experience in the summer perhaps and have fun from time to time.