I use Devonia Ltd in Buckfastleigh, South Devon. They do an excellent job and are very helpful on the phone to new customers. They have a web site and their no. Is 01364 643355. Basically, they will tell you to lay out the skins inner side up, trim off excess residual fat and straggly bits and spread a kilo of table salt all over it. Leave for 24 hrs and repeat the salt treatment. After a further 24 hrs, fold each skin wet surface to wet surface along the 'spine' line ( or put two skins together) and folc again. Then pack them into waterproof feed bags and send them off.( or take them, depending where you are). On arrival, they will be assessed as to their suitability for processing, so ring them to find out after a couple of days.
Now for the bad news. Because there are so few tanners now, they have a huge backlog. When I rang them yesterday, the waiting time is currently 8 months. But you will be amazed at the transformation from what you send to what you get back.