SomedayinUK,
Cough cough.. buy the laptop in the US, unpack it, cough cough, load up some known to you software/documents cough cough, walk through the airport with it cough cough, land in the UK, walk through customs nothing to declare, cough cough - job done. IF you get stopped and they do check your laptop, it'll be a used item as it has your documents on it and they'll wave you through.
As mentioned in other posts, this is 'grey importing' and whilst 'frowned' upon by the HMCE /VAT people - there's simply way too many travellers for them to stop and check everyone.
So long as you dont look like a walking personal items electronics store, you'll be fine.
*Disclaimer - I am simply providing the above info for information purposes only!!!!*
DtM! West London & Slough UK!