The thing about UCL is that the university is good, but the Maths department is less good in comparison to the university. So people assume that the Maths department is "very strong", when it's, say, only "strong". They don't have a dedicated Maths building (or, didn't when I applied!), so I don't really think that Maths is an overall concern of the University. Both of these things will impact both the funding and the resources that the department has or can offer, which will reflect in the teaching and research quality, and all of these things make up the overall standing of the Maths department, even if it doesn't affect it's reputation.
You've got to look past the reputation of a department when deciding whether it's good or not. Reputations can be completely outdated, and you've got to look seriously at the quality of the course. The university will provide some context, but there's usually great scope within that. For instance, Oxford's reputation for Maths isn't, perhaps, the greatest. And yet, it's quite a high quality course compared with the rest of the university. Perhaps undeservedly, Oxford is usually put a definitive second to Cambridge, when the courses are nigh on identical for the first 3 years. Part III at Cambridge is more different to the Part C at Oxford than the first three years, but again, you'll probably be well served whatever your chosen path of study is at either University. I can't even count the number of times I've, or my tutor (who studied at Trinity, Cambridge) have had to explain to people on Open Days who've just come out with 'Oh, Cambridge is miles better for Maths, though, isn't it?' that actually that's far from true.
Regardless, you'll get a top-class education from any of COWI.