All are strong for comp sci. In terms of grad prospects, I would go for Warwick. It is a target for investment banks, and given how lucrative the field of investment banking is, being a target really shows that Warwick is regarded very highly by employers in all industries. Durham's entry requirement is A*AA, St. Andrews is AAA, and Warwick's is A*A*A, so you would really be surrounded by the 'smartest' minds at Warwick if that's what you're looking for. But again, all are great for comp sci and congrats on your offers.