What the point on doing a joint honors in math and CS. When CS already has math's

The title. Not sure which one I should do.

The kind of maths covered in a maths degree (and hence, in the maths half of a joint honours degree) is very different to the kind of maths you'll have done in school, and a lot of it is quite different from the kind of maths done in a CS degree. Also unsurprisingly you'd end up doing more of it and in more depth.

But the maths half of a CS & Maths degree will not be mathematical methods similar to what you do in A-level, but much more likely to be a lot of abstract maths (abstract algebra, analysis, combinatorics etc) and maybe some applied and applicable mathematics (dynamical systems, mathematical statistics and stochastic processes etc). The abstract maths will be resolutely proof based usually, while the applied and applicable maths will usually also include a fair bit of proof based content, and the problem solving style material will still be a lot more open ended and abstract than what you've covered before (and often the emphasis is less on exact numerical answers compared to demonstrating whether a solution exists and is unique).

Unless you are particularly interested in that very abstract, proof based maths - essentially the "theory" of maths - it may not be something worth specifically pursuing as a joint honours. If all you want to do is get a CS degree then work as a software engineer code-monkey then it's pretty much entirely superfluous (also frankly in that case even a CS degree is arguably overkill and you might be better off just looking for degree apprenticeships in industry to start doing the software engineering and coding much faster, and a lot more of it).

So if I do end up doing a cs degree and later on decide I wanna do a job related to something finance could I still do it?

Outside of quant roles, finance does not require any specific subject. To work as a quant usually you need a PhD in a numerate subject (which includes CS - in fact CS is debatably more useful than maths as a background for that as it's much more algorithmic than theoretical I gather). To just become your average analyst at Goldman Sachs you could do any degree - you could do history of art if that's what interests you - provided you do it at a target uni.