Where are you gonna store it? In your backyard?
The idea is not to re-invent the concept of a gas station. Obviously you need an underground 10000 litre double wall gas tank and a pump, and the incorporated version of the flying club basically becomes an avgas provider at the airport.
The £0.35 discount or £ 3500 / fill would be higher than most low volume margins though, which would mean it wouldn't take too long to recoup the investment. You would need some margin, though, because you're not making any additional sales through lubricants, maps, drinks, snacks etc... like you would with a normal roadside gas station.
My question would be : what is the normal margin / discount that an FBO in the US would get on an order of say, 2-3000 gallons of avgas (which it sells on to the customers for $5.80) and how does that compare to the discounts that englishal is referring to in the UK ? As I said, I believe the £0.35 to be on the high side, but not exorbitant.
If its comparable, then we're back at the question of why pre-tax high volume delivery prices are relatively higher in Europe. If its not, then it makes sense to apply for an airport avgas concession with your flying club.