I've found history to be a massive jump up. The workload is literally like taking 2 seperate A-Levels. The quality of essay they expect from you is much higher as well. It takes a while to figure out what they truly want from you. I started the year on Bs and low As, but I now get 20+ marks out of 25 at least. Source questions in particular change quite a bit.
However, English lit is definitely the worst. This year I've had to endure a full play, 2 lengthy novels and 28 stupidly long poems. It's also much harder to do well in. I'm unsure as to whether it's my teachers' marking or what, but I was one of the best English students in the year in my secondary school, yet everyone in my a level class cannot get above a B in marked essays no matter how hard we try to improve. I went into A-Levels with English Literature as one of my favourite subjects, and it's now my biggest regret in terms of subject choice.
I've also heard that the sciences and maths in particular are a sizeable jump up.