Hi there! I took the exact same options for my AS levels : History, Physics, Maths and Further Maths so I hope I can give you some good advice on history!
The biggest thing, in my opinion, is to check what topics your school will be covering. E.g. mine did Tudors and Russian Revolution. I'll be honest with you, Tudors made life so dull, and I just couldn't handle it anymore, so I had to drop it. So you have to make sure that the topics you are doing interest you, since trust me, you will be having to read a looot of stuff, not just the textbook.
Secondly, history took up the most revision time by far. It has a very different style to revising than Physics and Maths, and that reflected in how long it took to revise. You have to read around the textbook, find out facts and figures yourself, plus ace your essay structure. It's definitely the most work for me.
BUT, so that this doesn't all sound so negative, I loved the Russian Revolution side of the course, and this was definitely great, since in a way it was a nice kind of break from doing just maths constantly. It was nice to be able to write some essays about a topic I really really loved, so that is also another good thing about taking history with those subjects.
All in all though, my advice for you, is to take a subject that you really really
love. Trust me, just because you are good at history right now, doesn't mean you will excel with it at A level, you really need to have a passion for it, a passion for researching about things outside of the textbook and a passion to write, write and write.
(also, with regards to Comp Sci, the A level is designed to me taken without the GCSE prior, since a lot of schools don't offer it as a GCSE, so if that is what is worrying you, don't be put off by that!
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