I can buy diesel for 105.9p a litre near me but the queues are enormous. There are a number of web sites telling you where the cheapest petrol and diesel can be found and it looks like a lot of people are using them.
On the subject of cooking oil, its sunflower oil that they run on. All the taxis are running on it, its legal now but you have to register with Customs and you are allowed to use 2500 litre a year before you pay duty.
You mix it with 5% white spirit, give it a good stir,and leave it to stand for 2 weeks.
Start by running on about 10% veg oil to 90% ordinary diesel and then just keep increasing the percentage over time till you reach 50-50.
The taxis do hundreds if thousands of miles a year on the stuff and the general view is that its better for the engine.
The only time you can't use it is in really cold weather because it goes too thick.