If you really think you have the motivation and talent to do a math undergrad, then I'd suggest going to a lower rate institution for your bachelors. Math is math; a Taylor's series expansion is the same in the UK as it is in the USA. The disparity between a math major at Cambridge versus London Met is smaller than between other majors. You are not relying on the teacher's experience as much, because math has such little ambiguity relatively.
Also, it seems that the job requires a master's, at least from all the US jobs I come across. That being said you would still need to get a master's. Do extremely well in undergrad and then apply to the high prestige places for your masters.