I am currently studying a bachelor's in (joint honours) Mathematics and Computer Science in the UK. I am intrigued by the sorts of problems present in theoretical computer science and I want to entertain the possibility of pursuing a PhD. However, I must first of course pursue a master's degree. The issue is I cannot find a master's degree with a focus in theoretical computer science in the UK (if you accept the notion of a master's degree being a specialist degree). In turn, I am not sure what sort of courses I should be looking for that would adequately prepare me for this field.
After browsing this forum for some time, I have found a mathematics background is paramount to theory. I will have what one may consider a formal background in mathematics by the time of graduation but I am not sure as to what exactly I should be doing. Would a pure mathematics master's be the best route?
Any insight or clarification into this would be appreciated greatly!