Gone into second year now, and personally I would say none of my subjects (Maths, Further Maths, Physics and Chemistry) required anymore work than at GCSE. If anything, I revised less for Physics and Maths than I did for GCSE. However, it is completely personal. Put the work in during the year, keep up with homework, and revise efficiently and you can easily pull off straight A-A*s, (this is in the more theoretical subjects), because at the end of the day it (sadly) comes down to how well you can regurgitate. A lot of people may feel differently to me, and it is purely opinion, but that's my view.
In terms of anything essay based, the workload seems much heavier. I get an assignment for maths every 3-4 weeks, and have to just keep up with notes in my sciences, and I get by on homework. However, I know people taking essay subjects (such as History) who get an essay per week, which for me would just bore me to death. History is great, and I would have taken it, given the opportunity to take 5, meaning I would have taken 4 of my 5 A* GCSE subjects on. I would really recommend taking maths, as it is a nice standard A level that if you do well in, looks good. After that, it's up to you. I would be biased to say TAKE PHYSICS, IT'S AWESOME, but hey, I like Physics.