Chris -
If you buy your laptop in the US and take it back to the UK then HMRC will tax it when you arrive. Basically you pay an import tax just like a vendor would pay to import the laptop and sell it to you there. Buying in the US may not be an inexpensive option especially if you also have to pay to swap out some keycaps and change a plug.

When I travel to the UK I carry paperwork to show US Customs when I return to prove that my laptop and software were purchased in the US and don't need to be taxed.

I'll second Bernard's of Toshiba laptops BTW. I've never had a bit of trouble with mine.
- Phil