Every Government department has a method of calculating values of everyday items. IOt seems that on this occasion they are using a John Lewis Catalogue. Could be worse ..... might have been Harrods.
It may interest you to know how HM Customs calculate the value of items imported. They look at a book of values and they look at the declared value (Receipt/Proof of purchase etc). They then take one from the other and charge you the difference. And the books they use? A standard Mail Order catalogue from Littlewoods and the Argos Catalogue.
Obviously this applies to items purchased for instance in the US and "over your personal limit" and to gifts sent to the UK from people resident abroad. I am not, before some pedant points out that Littlewood don't sell Weapons or Cars or Aircraft, talking commercial imports but the everyday items that they pull out of your case at Heathrow or they find in parcels marked "Gifts", for example, as they go through the Royal Mail.
For instance mail from Hong Kong all comes into the UK via Southamton Post Office where it all passes, on a conveyor belt, in front of one of a team of 12 Customs Officers who select parcels at random to be opened and checked against their declared contents.