One says, why should you pay a supplier to 'check' something they are supplying to you.
The other, pragmatic, mind says - they don't open the box normally and the laptop could be a dud. It might have dead pixels. One such service I saw also updated Windows and Virus software and would install any software you bought as well. It depends if this is worth 20 quid to you. I'd probably do this myself and take the risk on dead pixels.
From a retailer's point of view I can state with confidence that any e-tailer in the UK must comply with the Distance Selling Regulations Act which effectiveyl gives the customer 7 days from date of receipt to cancel their order for any reason.
This would allow you to forego paying for a presales check since if you were unhappy with the laptop upon receipt (for any reason) you could simply return it within 7 days - regardless of any dead pixel policy in place.