The PSU rating may not be an issue, all my laptops have universal 100 to 240 volt PSUs.
On the question of Customs knowing whether it originated in the UK or not,
1 it will look nice and shiny, very new, and he will probably have all the handbooks a software original discs with it.
2 They will ask a few questions, if he looks a bit on edge, they will ask some more, and some more.
3 They (probably) have a database of serial numbers of equipment which have been legitimately imported into the UK, if it is a laptop which is not manufactured/assembled here, they will become suspicious.
He could of course dress up smartly, and look like someone respectable, dont look shifty, and chance his luck just walking straight through the "Green Channel".
If he can get away with it - OK, if not prepare to be taken apart and examined in fine detail and if you are lucky, just have to pay the VAT.
Note - I am NOT RECOMMENDING this as a good course of action to take!
JimB