Hi there,
With the amount of personal electronics passing through all the airports in the world each day, it's simply impossible for everyone to be checked. Can you imagine everyone with a digital camera, ipod, digital watch, blood pressure monitor, laptop etc being asked to prove if it's new or not?!
Technically, yes, any new item bought from abroad and brought back into the UK is subject to VAT and/or import tax. If you want peace of mind, then you may declare it and pay the relevant amount.
The authorities of course know this, and will perhaps stop someone on the off chance and they 'may' have new electronics - in that case they'll simply take it on case by case and if it turns out they do ask you for the taxes, they will do that - if the item is obviously 2nd hand - they'll probably let it straight through.
Getting new electronics abroad and then bringing them back into the UK like this is called grey importing - the authorities knows it goes on thousands of times each day at every airport - they usually like to target those people who import/export on a much larger volume.
So overall, don't worry, get the laptop from the USA, take it out of the packaging, use it a couple of times and then fly over with it in your hand (in a laptop bag!) - you will not be asked about it at all.
Hope this helps!
Cheers, Dennis, West London & Slough UK!