It should be quite easy to switch between courses, I know a friend mine switched from maths to maths and stats after he did his final exam, he had done enough stats so he could switch. He is now working at RBS, not sure what he is doing there, but he is doing pretty well. You should be able to decide which course to do at the end of the first year or the start of the second year, as I think everybody does the same first year modules. This may have changed since I was in my first year, so it might be better to ask.
If you are to pursue finance in the future do the maths and financial maths, it'll put you in a good position if you pursue a masters although you will be repeating some units if you do the MMath. If you look at the course units in the MSc in mathematical finance and the MSc in Quantitative Finance and Financial Engineering, 5 of the modules can be taken in the MMath course. I did 4 financial math units, and to me they were by far the best units I took, and I think because I liked them I found them so much easier than the ones I didn't like, my lowest out of the 4 was 85%.
If you take the martingales course in your third year, when the lecturer, Tusheng Zhang says the module is very hard, don't listen to him, he just likes to get the lazy students out, so the average mark of the class is higher. The exam was the easiest I has throughout the course, and you will do a lot of exams, I did 42 over the 4 years, and when you include the large amounts of coursework, in-class tests, dissertations, group work, etc. Its definitely not a place where you can relax.
Whether you'll be able to find a really good job at the best places, you will have to have very good work experience to overcome the people applying from LSE, Oxbridge etc. It is possible for Manchester graduates to get into the top banks, I have a friend who is at GS, although he took computer science rather than maths.
I have been unable to get a place on any graduate scheme, and it has been extremely frustrating. I finished pretty much at the top of the year and to be faced with rejection after rejection is very disheartening, so it looks as though I'm going to have to spend 20K taking a masters at hopefully LSE.
If you have any more questions about anything, send me a PM.