I think you'll fine Exeter is less white than you fear - for example, there's a huge Middle East Studies department, which has plenty of overseas students from the region (OK, I know that's not South Asia, but it's evidence of ethnic diversity) as well as non-white British students: and that's only in one department. I really would try not to worry about it: I think it will be a non-issue unless you want it to be. If you do want to transfer after the first year, some universities will accept you (not all, though, and some will make you do their first year again) but you would probably need to justify it some other way.
Honestly - I'm sure you'll be fine: like I said, you won't be as alone as you think you will, firstly, and secondly, I'd be very surprised if anyone makes an issue of it.