His desire to stay at home is leading him in a very wrong direction in terms of optimizing his future potential to get a great degree and have career opportunities.
I went to Cambridge, and it's true that it has a lot of cachet, it doesn't mean other people can't do well from all kinds of universities, but honestly, if he has a Cambridge degree, then it will open doors for him especially if he wants to be a scientist.
I think if he is bright enough to get onto the Cambridge course, he may well be frustrated in a situation where many of the students are not very intellectual/motivated, I find that with some of my brighter students and I'm at a (not top 5) Russell Group uni. You are better off going with similar type people, it seems to pull them all along.
Imperial is also an excellent uni, but I'm not sure of any commute, lectures often start at 8.30am these days.
It seems such a shame he's limited himself like this, I wouldn't be happy for one of mine to pick Essex (or a local uni) over Cambridge and I'd be fairly pushy about not supporting that decision, unless I felt there was some reason Cambridge wouldn't suit them (which doesn't seem to be the case here, it's more a money/commuting issue).