First year Maths student at Imperial College Watch
I don't know about maths, but for my course (aeronautical engineering) we didn't require any books before uni started.
Reading list are given out by the lecturer at the start of the course, and also appear on line via the "Blackboard" system and on the Library website ....
However, unlike US universities, its very rare for a UK course to directly follow a single text, lecturers (particularly at a place like Imperial) much prefer to define their own course content and may well recommend texts for reading around the subject, but problem sets and worked answers etc are always handed out direct rather than saying "do problem 1-3 from book x". Multiple copies of recommended texts are also kept by the main library.
Its best if a student discusses with the lecturer and ideally some older students before shelling out on text books. I found that it was very useful to have a few key texts immediately to hand, but given the cost you cant buy everything so need to be careful with what you spend.