Computing Admissions ExamWatch
Don't think you can really prepare much for it, its more so skills you pick up. What you can expect is some maths questions and some programming ones, as in they give you pseudocode and you have to answer something about its outputs given inputs. For the maths if you have been practising UKMT/STEP/CTMUA questions etc. that should help and maybe just brush on on pseudocode and even a programming language as you can just write up the code they give you and run it yourself :P. Goodluck!