There is a difference in the legislation applicable to MOT standard for index plates, and Legality for road use
This comes about as specification for Index plates come from the 'Road Vehicle Registration and Licencing Regulations'. 'Road Vehicle Construction & Use Regulations' and 'Vehicle Lighting Regulations'.
As if keeping the right side (acceptable side?) off the law isn't difficult enough at the best of times!!!!
Basically, in order to satisfy the MOT test, the only requirement is the actual fitting location, height and angle of the plate, and that they have to comply with BS145(a).
BS145(a) only relates to the 'reflectivity' of the background material of Retro-reflective plates (as fitted in UK since 01/07/1973) - for a plate to conform, not only doeas it have to comply with the BS, but there HAS to be the BS145(a) logo in black on the plate itself (usually just under the last digit of the index number).
The size, shape etc of the lettering is not part of the MOT test. (unless it's really stupid so it can't be read).
However, just because it is MOT passable, the other regulations state that the letters WILL be of a certain script/size/spacing.
Things to be aware of:-
1. If Plod stops you and says your plate is illegal, you may get offered a Vehicle Defect Rectification Notice (14 day Mender) - this gives you a maximum of 14 days to get the car repaired and presented at ANY MOT testing station. As long as the noted defect is satisfactorily repaired, they stamp the ticket, you post it off to plod, and that is the end of the matter, if you fail, you get a day in Magistarates Court!
However, as stated, you COULD get a ticket for illegal spacing of the letters/numbers, but as long as it is BS145(a) compliant, the MOT station HAVE TO stamp it up as "legal" as it reaches MOT requirements and you avoid prosecution
2. If the same Plod stops you again, he'll probably think you're taking the pi$$ and give you a £30 Non Endorsable fixed penalty notice, even though you've complied with the 'legality' of the scheme (but not really the spirit of it).
3. If you have a Cherished Number on your car, and you deliberately mis-space or 'alter' the script to make it read as something else (Specky - are you reading this

) there is an option for the DVLA to take the number off you, without having to give you a penny for it, and issue you a normal age-related one.
Now, we all know that this doesn't happen (very often), and a common sence approach is usually applied.
That said, I've had a few numbers rescinded by DVLA 'cos the 'Boy Racer' has taken the pi$$ in extreme circumstances

On one occasion, the little darling lost a plate that his mum had paid £3500 for
Most of us adopt the policy that if it can be read and understood with a bit of thought, then it is okay(ish), and words of advice are usually enough to get it changed enough to keep everyone happy.
Eg, the odd white/yellow/black screw isn't a major problem to make a 4 or an 8 into an 'A', a 7 into a 'T' etc etc, and often a 5 can look like an 'S', but when you get some that you can't make head nor tail of, we get a bit upset.
Lastly, your mate at the MOT testing station MAY be prepared to give you a ticket with a duff plate on, but if we find out, then he will lose his licence to test vehicles, and his boss can get a visit which culminates in him being out of business for 14 days on first offence - i know it may sound harsh, but the VOSA take a dim view of rule bending as it shows a level of dishonesty. Conversely, if they miss a critical safety fault on a vehicle, then they usually only get words of advise as everyone makes mistakes - as long as it is done honestly
