When we're touring about the UK in a car we just buy all that we need from a supermarket and make our own sandwiches every day. Just carry a plastic plate a sharp knife and some cutlery. Tea bags, sugar, coffee even instant soup come in handy as we also take a flask and fill it up with hot water before leaving our accommodation in the morning.

Not to save money but for convenience. As it means you can stop and eat where and whenever you want.