Well, I have to admit to having a private plate, I also have a slightly different font from standard HOWEVER, the font used on my plate is one of the fonts approved for number plates and the spacing is completely normal.
I was stopped and given a producer for my plates about 8 years ago (yep I've had the same plates for a lot of years) I disagreed and told the officer I intended to take this further my number plates conformed to BSAU 145A regulations and the plates were stamped to that effect and I wasn't happy.
I took the car to a police station local to me without changing the plates (having been stopped in Skegness) and the traffic officers there told me there was no problem with my plates, if however I wanted a categorical answer I could contact Hinchingbrook traffic control who would inspect the car and give me a definative answer, I did that and got the same answer in writing!
I sent a copy of those letters to skeggy police station and got an apology from them, they admitted their officer was "over zealous" and they were happy to accept a police inspection findings.
On ONE occasion in the past 18 years or so I've had these plates I had a mot station question my number plates, I took the letter to them (Obviously I am not so silly as to not keep it with my driving documents) and they also backed off BTW I ALWAYS have my car MOT'd as early as I can, usually 4 weeks prior to requiring a new MOT just so I can deal with anything that might come up and not "have" to get the vehicle repaired at the MOT station.
I did by the way spend some 7 or 8 years working in the motor trade in retail dealerships (Vauxhall, VW/Audi, Leyland, Nissan, Alfa Romeo, Isuzu to name but a few (I was a group business manager) and have to say there were a number of occasions where I saw questionable decisions with my own eyes, usually when business was quiet! It may well be the MOT testers had more time to do a more thorough job, but on the other hand....
More recently (a couple of years ago) both my wife and my son both had MOT failures by the same testing station for brake pipes, in my wife's case her brake pipes had been replaced the previous year but it was failed (Yes I did complain and the owner said it was a judgement call and the tester had felt they were questionable) I had the pipes replaced (NOT at the testing station!) and got a pass certificate for the wife's car, my son however was really angry, took the car to Kwik fit, who are notoriously strict and paid for a second test even though it may have been just as cheap to have the brake pipe replaced, they passed his car first time!
We now use a different MOT centre.