Hi, I'm also planning to study maths at university.
1. It depends on the university you want to apply to - you will obviously have to be very good to go to Oxbridge but other universities have lower requirements. You should be able to meet the A-level offer requirements for the university you want to go to, and also be able to do adequately well in any admissions tests (STEP, MAT and TMUA are a few tests required by some of the top universities).
2. Maths at university is very different to A level. At A level, the focus is mainly on computation (you may have to calculate the value of a definite integral, or work out the equation of a line, or find the solutions to a system of equations). There's barely any proof in A level maths (or even further maths) but at university there's a lot more focus on proofs and rigorous definitions. You'll notice that all the A level calculus isn't really proved rigorously beyond saying 'this happens as x approaches 0' but at university in a course called analysis you'll learn to prove a lot of important results in calculus.
For example, when you differentiate a function you're implicitly assuming that the function is continuous and differentiable on a certain interval, which roughly says that there are no sharp points and you can draw the graph without taking your pen off the paper. At A level you don't move beyond this definition, but at uni you'll prove the definition of a continuous function:
A function
f(x) is continuous if and only if
Unparseable latex formula:\[ \lim_{x \to c} f(x) = f(c) \]
You can think of this in terms of the graph
y=f(x). This roughly states that as you approach a certain value of x (call this value c) from either above or below, the y-value will get closer and closer to the actual value of
f(c). A function such as
tanx is
not continuous since for example if you let x approach
2π (or 90 degrees, if you prefer) from below, the value of the function approaches positive infinity, whilst if you let x approach
2π or 90 degrees from above, the value of the function approaches negative infinity. Neither of these are finite numbers and are not the same, so this means the function is not continuous for all values of x.
3. It depends on the university - some offer computational modules but they shouldn't expect you to have any prior programming knowledge before you start the course.