Interestingly, though much of the content is the same, those domains resolve to different IP addresses, not just the same server tweaking the page for different domains as I thought.

Non-authoritative answer: Name: www.spectaclecases.co.uk Address: 83.170.88.134

Non-authoritative answer: Name: www.aglassescase.co.uk Address: 83.170.70.106

Presumably they have two different webservers, perhaps connected to the same ordering system/db server. I guess they could be using a virtualisation service and found this easier to set up.