I have 15mm spacers on the front of mine and 20mm spacers on the back. They fit directly on the hub and the wheels bolt on to the spacers. My front tyres are 18 225 40 and rear 18 235 40. That's because the front and rear wheels are the same size. If my rear wheels were staggered I would have got 255 40 18s.
Got mine ordered and fitted by FAW in hampstead London