It depends what you want to do with the degree, and what are you're planning to go into work in. You should also consider your financial situation, as while integrated masters are funded as part of the undergraduate degree with maintenance and tuition loans, standalone postgraduate masters are funded separately, and you just get one loan of up to £10k (possibly slightly higher now due to inflation etc) to cover tuition and/or maintenance and you see fit. This is particularly pressing if you wanted to do a 1 year masters in e.g. London, where living costs are extremely high.
However some (maths) PhD programmes prefer you do their masters course as a prelude to it (e.g. Cambridge). Either way, it's normally fairly straight forward to move from the integrated masters to the bachelors in the first two years of the course (and sometimes in third year, early on), and by then you'll have a much better idea of the area/career you may want to pursue and be able to determine whether the integrated masters or a standalone would serve you better. I'd suggest waiting until the end of second year, and revisit the issue then.
In most cases though, I don't think it will make a difference if you do the integrated masters or not. Unless you want to change field entirely (e.g. do a finance masters after a maths undergrad) or go into one of a handful of PhD programmes which normally expect you to do their masters course, and they don't have funding for a 1+3 PhD type programme, it's probably just splitting hairs.