I used these guys , I think a small double is £1200 , I've had mine a few years, and had a raft base , but it's got a year or two left in it. Hopefully last til I buy a place and can put a concrete base in, can't fault them for the money , think I paid about £800 so get wood is expensive as said above. Cheaper option would be a shed with pop holes, into panels...
https://rhtkennels.co.uk/