This is not based on any experience or knowledge but...
There is a fair amount of movement in the first semester of uni, plenty of people drop out and move courses. Considering you now have a better idea of the uni life you'd be happier with, it might be worth getting a list of uni's that meet the criteria and contacting them. You can be sure that they will all have people dropping out and may be happy to have you transfer over - especially as it's still the first couple of weeks.
I know you're thinking of Exeter, but there are plenty of small/campus uni's out there. Surrey uni is not far from London, campus based and Guildford is a nice, small town/city.