Does the MMaths mean you have to take a Maths masters. What happens if you decide pure maths is not for you, and you might want to shift towards something more applied like actuarial science, stochastics, or econometrics (or something engineering or physics based).
You can always transfer out of MMaths onto BSc programmes and finish after three years. You can transfer from BSc into MSci, provided your grades are good enough (min 2:i, often First required). There is a huge advantage financially in terms of integrated Masters - these are financed by undergraduate loans. A separate Masters can be financed (partly) by postgraduate loans but this increases the percentage of salary taken in loan repayments as postgraduate loan repayment is added to undergraduate loan repayment.
I would strongly emphasise that UK Maths degrees are NOT just pure maths. Over 2/3rd of the Cambridge Maths department, for example, does NOT do pure maths - they do stats, actuarial science, management science, financial maths, industrial applied maths, engineering maths, mathematical biology, quantum computing, scientific computing, theoretical physics,.... (Remember that Stephen Hawking is based in the Cambridge maths department.) So an MMaths in a top UK department already gives you access to a huge range of research areas/career directions. In the fourth year at Warwick, Cambridge etc you can specialise in engineering maths or mathematical biology or quantum computing or whatever.