I have shot weddings in Cambridge and there are loads of locations. Back of kings is an obvious one, but also go the back way into St. John's (near to kings) there is a huge lawn and nice buildings, views onto the river and the bridge of sighs. The front of kings is very busy with tourists so best avoided. You could get a punt and get shots on that too?
There is fitzwilliam museum which could be used as a location, and also a nice roof terrace of a bar (varsity hotel) which has great views. There are a range of different colleges, I quite like Pembroke, but some May have restrictions on entry so best to check beforehand.