Mostly wherever there's room, probably. They will also be trying to fulfil quotas, so that no one college has all of the biologists etc. . But I don't think they really use your personal statement (if you had a preference, you'd probably have expressed it by choosing one already)
As far as I know, it's entirely 'at random'. I don't think rooms are allocated at all until almost all offers are resolved.
Theoretically you can probably swap, but it would be at the discretion of both involved colleges if so. It would be easier to just make a choice that you like in the first place
If you're in a catered college, you pay for all your meals (though you can get a small refund if you opt out of some, not sure how flexible that is). As such, making your own food is kinda a waste of money. The option isn't really available anyway, as you almost certainly won't have a kitchen beyond a fridge and microwave.
Also, St Cuths has some self catered accommodation if that's what you want.
If this information isn't on the uni website, it will probably be on the TSR wiki or in one of the many other threads about this topic.