There are two major issues and two minor issues here.
Major 1
Although laptops are designed for travel, many US citizens only travel within the US. Consequently it is not uncommon to have a 110v only AC adapter. You need to physically check that the AC adapter is 'voltage seeking' to allow it to work with UK mains voltages. It should read something like '90 - 240v AC input'
Major 2
Warranty is an issue if you buy a less well-known brand. If you buy a well-known international make, then it will come with an international warranty - check. If the laptop fails completely, you can take items purchased in Wal*Mart to any ASDA for a refund.
Minor 1
Any integrated DVD player will be for Region 1 DVDs. Check that this can be reset to Region 2 for UK DVDs (Sony can be particularly difficult in this respect)
Minor 2
The US bought laptop will have a US keyboard. Whilst the character mapping can be reset, the keytops cannot. Until you get very used to it, you will make mistakes.