You will have no problems. What I would suggest doing is not bringing back the box, as then you could say that you bought the iPad in the UK and took it on holiday with you. Yea, I know you should really tell customs that you have bought an iPad, but I'm not sure how much they would charge you to import it.
If you sign into the iTunes store on the iPad with a UK iTunes account it will switch to allow you to access the UK iTunes store. You will not have a problem here at all.
From the $629.00 price you're obviously looking at the 16GB 3G iPad. As far as I am aware the US 3G iPads are unlocked and can be used on any network, which would say to me they are international and you can pick up a micro-SIM from any UK carrier and everything will be fine.
BUT, I would look for further clarification on this. I'm not sure what Apple's policy is on imports and whether you will be able to get support / return it in the UK if you find that the 3G doesn't work. (From a quick Google it seems like you will be fine, but I would recommend doing a bit more research as I wouldn't want to spend £423 on something that might not work over here).
Also, if you want to be fully legit with taxes etc. I'm pretty sure you can claim back the US sales tax if you declare your iPad at customs. You do get so much allowance on tech, so you might not have to pay VAT on it. Again, I would check here:
http://www.hmrc.gov.uk/customs/index.htm
Hope this helps.