We advise you to read the Computer Science subject page thoroughly.
Many Computer Science with Maths offers include particular grades in STEP examinations. You can find the Examiners' reports for last year and the specification for papers on the STEP website (in the links below).
For offer holders:
Those with offers to study Computer Science should consult the "Preparatory reading and practical preparation page" on the Computer Laboratory website (see the links below). There are further reading suggestions in most cases if you look at the 'syllabus' section for the different Part 1A courses.
There is also a Mathematics workbook which is written primarily for Natural Sciences offer-holders but is also essential for Computer Scientists. This is updated in late July each year and you can see the date on the coversheet.