Some fairly comprehensive advice from the RYA at
http://www.rya.org.uk/sitecollection...F%20PETROL.pdf
Seems to be one of those bureaucratic beauties whereby a 20 litre metal jerrycan is OK to "transport" petrol, but the max size for "storing" it is 10 litres... so if you fill a 20litre can for the boat but then don't use it seems you're supposed to transfer it into two smaller cans - or put it into the main tank if there's room, which rather defeats the object if the 20l was supposed to be kept as an extra supply for emergency backup....
Also note the regs about storing any amount of fuel over and above a couple of 5l cans for the lawnmower are pretty stringent in terms of what kind of building, walls to prevent fuel flowing out in the event of fire, etc. Stacking the garage with 20l gerries full of petrol is a definite no-no!