I recently bought a cherished number plate from DVLA direct - nice and easy process.
I bought number plates on eBay as it's a good way to do so when you don't have all your documents (they're a lot cheaper too).
I used the 05 as OS for my number plate and chose a "close to legal" font, but didn't have the space between the first 2 letters and the OS.
While travelling through Gloucestershire recently, I was stopped by the boys in blue for the spacing. They were supposed to issue an on-the-spot fine, but chose not to BUT they did record it and advised that if you get caught with illegal fonts or spacing more than once, DVLA is informed and can choose to withdraw your number plate.
If DVLA does that, you lose the money you paid for your number plate, no comebacks, nothing.
I now have a more legal set of plates as I didn't want to lose them.