I'm a US "American Apparel" size S... which is apparently a XS at Jay Jays, but a S at Dangerfield and an M at Cotton On.
AFAIK American Apparel's is accepted as a global standard size due to market proliferation.
congrats on owning a skyline. those guys who wanna overtake you but not go any faster just wanna prove to you that their car is just as fast, if not faster than yours. consider it penis envy.
The only way to get one cheaply would be to move to the UK, own it for a year, and move back.
Or pay someone you know is gonna be moving there for a year, and trust them with your car.