The aim of this protest is to send a message to the British government saying that motorists are not happy to be paying 75% tax on fuel. It is not aimed at hurting the petrol companies and it probably won't do so. As stated by TallRoo, motorists will still drive and therefore will just buy their petrol on the adjacent days. However, refraining from buying petrol on certain days makes the protest measurable. This way opinion can be made clear and everyone concerned gets a chance to do something about it.