Yeah, switching to the BSc is generally easier than the other way around. It shouldn't be an issue. Then obviously after completing the BSc you can apply to other unis for postgraduate masters degrees.
Just note you won't get funding from student finance to do a postgraduate masters, whereas you will for the undergraduate masters.
However I've read that undergraduate masters are actually worth 20(maybe 0) credits less than a postgraduate masters, so a lot of overseas employers won't recognise it as an actual masters degree. Obviously this point will only matter depending on your desire to seek employment outside of the UK after graduation.