I know there is a lot of hate for Lancaster on TSR but my mum went there to do maths and absolutely loved it, got a first, and now has a job with some of her coworkers having maths degrees from oxbridge. She didn't go to Lancaster because she wasn't capable of going to places like Imperial (or even Oxbridge, she is one of the cleverest people I know), she just loved the maths department at Lancaster, it's really small and cosy. You can still get great jobs by going to Lancaster, my mum certainly did, anyone with a first in maths isn't going to have a problem really. Lancaster is consistently in the top 10 for most university league rankings, it's 10th for maths (above LSE and UCL). So if you're excluding Oxbridge, London Unis etc then Lancaster is definitely worth considering. But it should just come down to you and if you'd be happy there; you're never going to do well if you don't like the university, the course, etc. No one can tell you the best place for you to go - only you can decide what's best for you.