Plenty of people go straight from BA to MA without ever stepping inside an arch office. It's dumb not to at least have an idea of what the profession is like, but that should not stop you. Plenty of time during the MA to learn.

It's probably a good idea to get some experience before you graduate with a masters though. Not only for the sake of having experience and being more marketable, but also to make sure that this profession is actually what you want to do-- architecture in an office is much different than architecture in academia so you should really make sure you know what you're getting into before spending a lot of time and money on your education. Try to do an internship the summer before you start school and/or during the summer breaks. Some schools also have co-op programs as part of the curriculum in the masters program like U of Cincinnati. You learn SO MUCH more working in an office to prepare for professional work. Having some experience will not only enhance your resume, but it'll put you at an advantage with also knowing what kind of firm you eventually want to work for-- big, small, medium size, what kinds of project types, etc. so there's a smaller chance that you're stuck somewhere that you hate after graduation.  

Be careful. there are many MArch's (12-18 months) in the UK that are not towards Part 2. There are no requirements for non-professional degrees and often interdisciplinary so they like taking candidates with other related degrees. 

I don't know any Uni in the UK who would offer a place on a Part 2 without a previous architectural degree (international or Part 1) - what university?