With regard to BP sites, I've nearly always found them to be a good bit more expensive than the rest.
Filling up at BP Hailsham the other day, there was a sign saying they didn't accept some BP card on lpg sales.
This was because Flogas supplied the lpg, hence the low price I suspect.
Mind you, if you want a real rip off, try buying a 6kg refill propane bottle.
That's about 12 litres and costs in excess of twenty quid, so over three times more expensive than pump gas, which I think is taxed at a higher rate, making it an even bigger rip off.