As far as I know, there is no restocking fee in the UK. Certainly not within 7 days if you've ordered online (covered under the distance selling act)
I'm considering doing the same, whist I only fill 50/60% of my 16gb now, I want to keep this phone for at least 2 years, so I'm considering returning it and ordering the 32gb...
Only thing that is stopping me is the long wait to get a replacement >.< I've not sold on my 3g yet because I've been unsure about this and I'm not sure I can go back to it, even for a few weeks!