Wheel spacers in the U.K.

[Wearethesound]

Anyone here know what size wheel spacers I can legally use on my R51 pathy? I'm from the states and was told there is a law about how much tyre can be visible outside the fenders? I did a google search and haven't found a measurement. Some sites say "no tread" others claim it's between 1/2"-1" roughly as it is posted in mm. I was going to order some 1" spacers front and rear or maybe 1" front and 1.25" rear. Help!?