If divided between 4 people, the weight would be about 2.6 pounds (amazon says box is 10.2pounds), and according to USPS, priority mail (variable weight) would be $24.25 per package
So for UK people, say $11 for 23 diapers, plus $25 for shipping, comes to about £18.00 GBP
Or I suppose you could just send a few diapers for a lot less, but then you'd need a lot of UK people (or just keep some for yourself )