you are legally responsible for maintaining any gas appliances in the property (even if the tenants agree to do it). But you can do what you like with white goods (fridge, freezer, washer/dryer) - it's normal that if you supply it, you maintain it, but if the tenants agree to it simply add a clause at the end of the AST. The only thing I'd say to consider before doing this is what would happen if the items were beyond repair, if the tenant replaces them out of their own pocket it's theirs to take with them when they leave