Dave mentions some very good universities in the UK, but if it is quantum computing you are interested in, then you should definitely add Bristol to the list.
Now, I notice you mentioned that you were interested in quantum computing research. I should probably point out that a lot of researchers in that field come from a physics background, rather than having been CS undergrads. Perhaps this is something to consider? In many of the universities, quantum computing can be taken as an undergrad subject in both physics and cs (indeed I believe it is more commonly offered by physics departments than CS ones).
My experience of UK universities has only really been Oxford, where I worked for a number of years, and where I did my doctorate. There I know of at least 3 departments (Physics, Materials and Comlab) that offer QC courses to undergrads. Research groups are also spread out over a number of deartments, with theory research in at least 4 departments (Physics, Materials, Comlab and Maths). QC is an interdisciplinary field, so there are people coming at it from a lot of backgrounds. I think perhaps it is worth thinking about what you really want to do: Is your heart set on quantum computing, or is it more generally CS theory or physics or mathematics that holds interest for you? Certainly people come to QC from any of these backgrounds, so whichever course you pick, you are not ruling it out. Personally, I did theoretical physics at undergrad, then did my doctorate in a physicsy quantum computing theory group, but my research now is a mix of physics and TCS.
If you do decide to go the CS route, one important thing to note is that not all CS courses are created equal, and some will be much more mathematical than others, depending on the university. Taking a course that is heavily focused on software engineering is probably not a very good idea for the type of future research you seem to have in mind. It is probably worth keeping this in mind when choosing where to apply to.