You can buy millers in a 5ltr container. I can't remember the exact price, but it does work out a lot cheaper than the 500ml bottles.
IIRC you need to be in the motor trade or similar to buy the 5ltr containers and I think the minimum quantity you can buy is 4 X 5ltr.
Johnsco is getting some 5ltr containers from Millers at Bradford, West Yorkshire, but they do have depots in other parts of the UK.