Quote From Baltar:

The 'best' option depends on your personal circumstances & your career aspirations. For example:

1) what do you need the MSc for? If you will go straight into employment & aren't thinking of doing a PhD, then it all depends on which university your potential employers will regards more highly. There are some universities which are highly regarded outside the uk, but in terms of your area of specialization they might not be ranked very highly. If you want a job at the end of it, a high ranking university is all that matters; but if you want to do a PhD then the department where you did your MSc could play a significant role in where you are accepted to do your PhD.
2) as the previous posters said, look online for the cost of living at both locations. London is expensive but then again it's cheaper if you want fly home regularly from heathrow or gatwick airport of your family plan to visit you regularly. London is far more cosmopolitan and a very busy city - do you prefer the calm and quiet of the countryside (Durham campus) or being bang in the middle of one of the busiest cities in the world (UCL campus)? Will London be too much of a good thing - I.e too much of a destraction, or are you quite s disciplined person?