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Well most worthwhile universities don't care about UCAS points specifically. They're a bit useless because you can boost your score by getting mediocre grades in more subjects. Also they don't differentiate between harder subjects. Unis have a list of subjects they respect, if your subject isn't one of them you're wasting your time flung it.
But yeah, if your grades are naff you probably want to look at doing a foundation year. A quick Google shows that Manchester offers one for compsci but you'll want to do your own research on what the requirements are exactly. I only know of entrance exams in addition to A levels where the subject isn't covered in enough depth at A level (e.g STEP, BMAT etc) I've not heard of it as a replacement for A levels.