Based on a brief glimpse of each university's course website I'd say the best would be at Hertfordshire based on the fact that you get some options in 2nd year although it basically becomes a choice between stats or no stats. The 3rd year seems to have a greater range of options than the other two as well. Totally willing to be disproven though. Middlesex appear the only one who offer a MMath though which could be important to you though it sounds pretty dodgy from what their website describes.
Also, I understand that 280 ucas points is BCC at A-level? If so, disregard this next bit. The standard entry into year 1 of some decent Scottish universities is BBB so if you could get that then Heriot Watt, Strathclyde and Aberdeen could be options. I'm sure there's a few English universities with better courses around that grade range too so I'd encourage you to aim as high as you can in this situation.