if there is a contractual obligation the customer pays £25.
Indeed there is no contractual obligation, unless you agree to it.
However, to keep in the realms of this topic, dealers add a nominal fuel charge as a courtesy to its customers and whils’t most dealers will ask up front, but some don’t. Likewise it is also up to the customer having due diligence prior to signing a contract and whether such a charge is clearly stated or agreed upon.
Either way the customer has the right to decline such a charge and/or award the salesman Clubcard points or similar.