I found a couple on ebay... might not be any good to you but you will know what's more suitable for you dog than me. Of course, they are not free and will cost you... both aren't in the UK either.

This one is from America but ships to the UK, not sure if the size is right or not:
DOG WHEELCHAIR ( J-4 MEDIUM SIZE DOGS) on eBay, also Health Care, Dog Supplies, Pet Supplies, Home Garden (end time 21-Sep-08 23:41:05 BST)

This one comes from China, obviously the one in this auction would be too small, but they do mention that if your dogs weight/size is different, then to contact them so they can put one in your size up for auction for you to then buy/bid on:
Dog Wheelchair Canine Cat Cart SMALL MEDIUM LARGE CUSTO on eBay, also Other, Dog Supplies, Pet Supplies, Home Garden (end time 22-Sep-08 03:08:10 BST)

They do different sizes by the looks of it, and do custom made ones... and also show how to measure your dog for the right fit wheelchair.