As someone who's just finished a Maths degree at Lancaster, I feel I have to stick up for it a bit... Yes, it doesn't have the reputation that Warwick does with the Russell Group aspect, etc., but I've found it to be a good department willing to support students.
Lancaster does have a maths society, but it's not particularly active as few people took interest in it - personally, I found it much easier making friends directly on my course and forming a much more personal, friendly study group that way. The different workshops for modules and projects you do also provide opportunity to make new friends on your course, for study or otherwise. There are also lots of other societies, such as the Investment Finance Society, which offer lots of career help. Also, that waving goodbye to investment banking opportunities if you go to Lancaster is rubbish, considering I have friends now heading into graduate positions within that sector.
Yes, there is a lack of flexibility in module choices in the first two years, as all modules are compulsory. However, I think I've actually benefitted from that, as I wouldn't have done any analysis given the choice straight away, so I've now got a broad level of understanding whilst then being able to go into depth in my third year. With friends having just chosen their fourth year modules (I'm going into working in actuarial, saw no benefit in prolonging exams by doing another year at uni), I'm aware that Lancaster are growing the amount of modules they're offering each year. Yes, a lot of what's on offer is stats-based, but, again, that was what I was interested in, hence why I've enjoyed the course.
Ultimately, it depends what you want to get out of university. I wasn't just interested in the prestige of a the university I was attending, I wanted to feel at home there and also know I could enjoy my time studying, so felt Lancaster was better for me. Plus, the minor scheme they offer in first year allowed me to study areas outside of mathematics. Under that scheme, you could minor in something like Economics if you so desired, and then even continue this into your degree in subsequent years if you wanted. That flexibility was of great appeal to me.