I would highly recommend Brunell for it's MSc in Integrated Product Design course. I've seen many of the top design firms in London recruit candidates from there. It's a great hands on course. You come out really well formed as a designer with exposure to all aspects of design including design management.
UCL has a HCI course but it's VERY academic. Employers haven't been too happy about that course primarily because it's heavy focus on academia. If your goal is to do a PhD and get deep into research this is the course you want. But if your goal is to get into the market I would not recommend it at all.
University of Greenwich also has an Interaction Design Course(http://www2.gre.ac.uk/students/international/erasmus/incoming/catalogue/cms/cms-courses?banner=COMP1649&cyear=1415) but I've not heard much about it.
Lastly, I've heard good things about RCA. The only thing I've heard students complain is the tenure of the course being 2 years. If you're willing to spend 2 years on your Masters, and are looking for a great name, and get exposure to a solid network this is the course.