Mmaths or BscWatch
I've applied to do maths with economics, is it better to apply for a Master or a Bsc??
In many ways, doing the 3 year BSc and going elsewhere for a one year MSc would appear to be a very good way of getting a more varied and stronger education, but as was pointed out it is a more expensive proposition. Obviously if you fancy a break or money is no object then that may not apply. The other cases in favour of the BSc are obviously the shorter time, which may be very important if it turns out that the MMath doesn't actually increase your career prospects more than having a year in industry behind you.
In short, I'd say get onto the MMath if you can and weigh up your options between now and when you have to change. The maths departments themselves aren't likely to care too much one way or another (here they just want you to achieve a slightly higher standard on your courses) but your LEA will I think.