You didn't say whereabouts in the UK you'll be travelling, Bob, so I think you should take a look at this....
then no matter where you are, you'll always be able to find the cheapest petrol in the area, providing you can get online!
You do need to register, but it's free, and you only get an email every three months or so (been a member for 2 years)
Be aware that all the stations are self service, some are credit card only at the pump (i.e. unattended) and that in the UK it is an offence to fill your car up without the means to pay. (not saying you would, Bob, just not sure if you have the same law in Boston! No offence!)
Also remember that if you're using our motorways, the service stations on them *always* have higher petrol prices. Try and get off the motorway for fuel - and food! Village food is far better, cheaper and fresher.
Have a wonderful time, Bob!