1. Maths degrees vary massively in their entry requirements, from A*A*A*+ STEP (one of the uni admissions tests for maths) to a minimum number of UCAS points (perhaps 72 or similar, which is CDE I think). A level maths is almost always compulsory, however some of the top unis either insist or heavily encourage you to do A level further maths also
2. Yes, although the top unis encourage further maths also
3. What you study will vary massively on the uni - if everyone is starting with A/A* in maths/further maths then obviously it will start going beyond that very quickly (eg. multivariate calculus, analysis, algebra (groups + fields, not what you think of as algebra!), dynamics, multivariate statistics and a lot more I've forgotten). If the most maths people have studied is they have a C in maths A level then they will start with the harder topics of A level maths to get that solid first. They may then go into A level further maths, or they may study some of the topics above - it all depends on the course.
4. At the top level, a maths degree isn't very like gcse/a level. It's typically a lot more rigourous, more proofs and concepts than any numbers involved. I suspect at a lower level there would be more work like a level work, but I haven't had that experience.
5. There's a lot more to a maths degree than enjoying gcse maths tbh. But if you don't enjoy gcse maths (for reasons other than it being too simplified as I know a few people who thought) then I wouldn't recommend a maths degree
6. A maths degree opens a lot of doors - there's not many, if any, careers which specifically require a maths degree, but generally numerate and logical careers (anything in tech/finance/teaching etc.) is open to you
7. I think this depends as much on the job market at the time of graduation - but in general a maths degree won't hold you back in the job market
8. This massively depends on what career you do
It could be anything from fairly low paid to very highly paid. I believe the average is quite high though compared to the UK average
Hope that is helpful!