<p>The best schools in the UK for math (called “maths” in British English) are Cambridge, Oxford, Imperial College London, University College London, and Warwick. St. Andrews university in Scotland is also well-known for applied math specifically. </p>
<p>Out of the aforementioned schools, Cambridge, Oxford and, to a lesser extent, Imperial are highly selective. You are unlikely to get into their math program straight out of an American High School, unless you have evidence of mathematical training at a level much higher than the usual US standard. AP Calculus BC and AP Physics with a score of 5 in the respective exams are probably a must, but may not be enough for a successful application.</p>
<p>Note that the normal time required to get a bachelor’s degree in math in the UK is 3 years (as opposed to 4 years in the US), but, at most school nowadays, including Oxbridge and Imperial, you may qualify for a 4-year extended course from which you will graduate with a master’s degree (called an ‘MMath’ at Oxford) or, alternatively, with a certificate of “postgraduate studies” (in the case of Cambridge). In both cases, that allows you direct entry into a PhD program in math in the UK (and possibly elsewhere in the world) after 4 years only of college study. </p>
<p>Another important point to know is that math degrees in the UK are highly specialized. Throughout the 3 or 4 years, math is pretty much the only subject you will study and at a very deep level by normal US undergraduate standards. There are many options available in applied math (some of which are compulsory) that will allow you take a fair amount of (theoretical) physics in addition to pure math, but, beyond that, there are no “general education requirements” (like required humanities or language classes) as it is the case in the US.</p>