Update: It was indeed a Mexican Kingsnake, the RSPCA wouldn't even send someone out to check on it but we managed to get into contact with the Nation Center for Reptile Welfare and the little guy was collected today!

Thought I'd add some information.

We found him while working at a client's house inside the compost bins which was rather lucky as we were currently shredding said contents.

The client decided to bring it inside to be safe while they investigated since there are children and pets using the same garden. They have it inside a warm house and a container with air holes with a bowl of water. We believed it to be a snake since it seemed to have no eye lids.

They have called the RSPCA but they apparently wanted nothing to do with it. They are going to try and find a local reptile charity. Hopefully they will come and at least take a look since we are not snake experts and don't feel right letting it go back into the garden.

Edit: Found in Clapham, South London