If you take a customers credit card to the shop and use it to pay for materials (with their consent) they are in breach of the card terms and by presenting yourself as them to the shop, you are potentially commuting fraud. They would be processing the payment card holder present, which is your deception. Further the card holder might later say you didn't have his consent, which might expose you to a theft allegation. As others have said. In the age of the internet, you can sit them down with an ipad and build up their bill of material and get them to pay or you ask for an initial payment to cover materials. Hopefully you'll find alternative to travis perkins who are over priced normally. However, your safe/legal option is to call the customer and get them to deal with the payment over the phone and allow TP to process.
In terms of establishing a contract. You can still function on trust/oral agreement, but you protect yourself if you write down, what they are asking you to do. e.g. supply and install 6M of wood fence with concrete posts to a height of 2M, remove and dispose of existing fence. Estimate 2 days at £200 per day + £500 materials (est) or Total price £1100 and either get them to email agreement or sign it. You don't need a signed paper contract, you can email them and get them to confirm they wont to go ahead.
You are a limited company if you form a limited company and you commit an offence if you present yourself as one when you aren't.
I'd get public liability insurance too.
Giving someone a kicking who wont pay a bill, will probably end up with you sitting in prison and not getting your money.
