Take a look at this document from the RYA website
This suggests that it is ok to transport fuel in Jerry cans and outoard fuel tanks.
At Spinnaker I store my fuel in a trailer in 25 litre outboard tanks (approx 250lin total). These can simply be put into the boat when required with out the need to keep pouring fuel from jerry cans. The trailer has a locking lid and is chained to the ground.
When I need to fill the tanks I simply tow the trailer to the garage and fill then up. I have a Fire extinguisher mounted on the outside of the trailer and carry a fire bucket (sand) in the trailer incase I spill any. I also carry a copy of this document in case anybody gets funny with me.
We have an arrangment at the local garage where we have an account that we can fill up this amount of fuel. I do get some funny looks from other customers some times when they see me filling up this many cans and I did go one evening and there was a new guy working there who got arsy with me and refused to sell me that much fuel into plastic cans and refused to come out and look at them or check the regulations or contact his manager. A quick word with the manager the next day resolved the issue and i assume the guy got a slap from his boss as he's been nice as pie since!!!
One think that may be worth checking is whether your truck is a 'car' or 'van' and this will effect how much fuel you can carry in it according to this document. May be worth a call to RYA Legal department.