Really does depend on if you happen to drive past a traffic/proactive copper...
I ran German font on my Golf for 3 years - MOT no probs, but the whole point is the ANPR system doesn't recognise it as a UK number plate...
So if you do get stopped, it can be up to a £1000 fine, and a correction notice that has to be stamped and sent off by an MOT station within 14 days
I think when I got stopped, they just issued me with a correction notice, and obviously you've got the cost of the new plates...
Not worth the hassle these days!!!