Apparently it is legal to purchase and transport up to 200 litres of petrol by road in approved containers, but the maximum you can store is 15 litres, other than in your aircraft tanks, without falling foul of the regs wrt signs, firefighting equipment etc together with authorisation from the local Trading Standards people. I use either 20l metal jerrycans or 215l oil drum depending on how flush I feel at the time. Never had a problem at my two local aerodromes with cans or drum, but have never gone to local garage with a drum, only cans...
No idea about whether or not a bowser makes a difference, although have stored up to 1500l temporarily in a borrowed bowser in the past (this was JET A1, however)
I'd be cautious about using plastic cans rather than the steel jerrycan types, especially on hot dry days. Never had aproblem with condensation, either but am careful to check tanks for water and yuk.
Anyway once you have done your own risk assessment and take very seriously the dangers of handling petrol (250 galls is a lot! Apparently equivalent to nearly 20000 sticks of dynamite), what the eye don't see the heart don't grieve over....If you're snow white and really want to know then call the HSE 01787 8811165 and ask for a free booklet regarding transport of petrol.......