I decided early during my undergraduate studies that I, in order to prepare myself better for a subsequent PhD, would get myself an MSc in AI, come hell or high water. My initial fear regarding my chances of being accepted by a reasonably reputable university in the UK quickly vanished after I wasn't rejected by any of my top choices, which didn't surprise anyone as much as me.
I now have to decide between the University of Edinburgh, the University of St Andrews and the University of Southampton, all of which offer a specific postgraduate programme in artificial intelligence and all of which offer course combinations which I would be more than happy to spend a year of studying with. This excludes the easiest way to differentiate between programs.
So, at the end of the day, it comes down to reputation. (Primarily due to the MSc being intended to act as a stepping stone to a PhD.) From what I gathered, the main difference is that Edinburgh and Southampton fare better in international rankings and are better known for their computer science departments, whereas St Andrews gains the advantage in national rankings and, while being very prestigious overall, is less known as a stronghold of computing. (I don't really trust the league tables in that they catapulted computer science at St Andrews from #15 to #4 from last year's ranking.)
To be honest, Edinburgh and St Andrews are prettier cities to study in, so Southampton already has a slight disadvantage. (St Andrew's tininess, however, is not a problem as I like small towns and prefer a lonely walk and a cosy pub with a couple of fellow students over a club with loud music every day of the week. That is, of course, entirely a matter of personal taste.)
I'm thankful for any opinion regarding differences in how those three universities are seen, both in general and for computer science. As the Continental chap that I am, I unfortunately lack the implicit insight into the UK's higher education landscape, so all help is welcome.