Yeah, you can start TMUA prep from now and have a decent chance. I think most people will have started earlier but you should still be fine. "What degree is better" is a very subjective question. Both have good prospects, if that's what you're wondering. It would help if you would be more specific. Uni rankings often contradict each other, but it is true that some universities are more prestigious in general than others. For some professions such as investment banking, university prestige significantly matters (for investment banking, LSE, Imperial, Oxford, Cambridge, UCL and Warwick are the best, but there are also some semi-target universities that are slightly less prestigious). However, university prestige doesn't matter for every profession. A good personal statement will generally contain at least 70% supercurriculars (things related to the subject you want to study at uni that you've done outside a levels such as reading subject related books etc - you can find many examples of these online, and general advice on the websites of the universities that you're interested in). Also, LSE Maths and Economics has very excellent prospects and earning potential.