They are pretty different, in terms of both what you’re learning and how you’re learning it. You probably study much fewer subjects – just 1 at a UK university – in comparison to studying 6 or more at IB. And, rather than days full of classes, you have much more independent study, broken up by lectures and smaller classes or tutorials.
However, IB definitely prepares you really well for the workload of university and learning to balance this with other activities. The EE is also a great step towards doing longer, more in depth essays and projects. I did more work (academically) during the last year of IB than first year at uni. The hours I spend working at uni are also much more flexible – it’s up to you how much time and effort you put into writing your essays, etc. That said, university life is busy with other things like societies, making new friends, and sport. In comparison to the pretty rigid structure of an IB timetable, it’s up to you to figure out how to spend your time at university.