3D 'style' plates are legal so long as they conform to the correct font which they look like they do. I take you have the BS number and name of the producer/postcode as well? The ENG/flag is a permitted variant so no issues on that front.
Can't find the exact legislation atm (I will find it though) but worth a print -
https://www.gov.uk/displaying-number-plates/rules-number-plates"Characters on a number plate can be 3D. "
As for the 'well they haven't told us" remark - that's not acceptable. How can you deal with offences when you don't even know the law. I've had the same experience as I run pressed, fully road legal plates. Got an apology out of it, especially after being told i'm also 'boy racer' but annoys me. It's no excuse.
One last point - although the MOT regs are pretty spot on with plates now, there was a period when legal plates were being failed and vice versa, so be wary of relying too much on the MOT manual - although it is an excellent read!
There is only one font that is now legal unless your plate is older than 1.9.2001 (I think) and then it has to be similar ie The letters have to be black and the correct size, spaced correctly etc. my private plate is a P reg (1996) so if I understand it correctly I can have 3D font legally as long as its the same size, colour blah blah blah.
If you mean because you have have 1996 plates, you can run as per the 1996 regs, you can't unfortunately. As you have physically placed the plates after 2001, the new regulations are the ones that govern your plates! I have a similar age private plate.....if you we could, i'd be running a black and silver
I think I may get a cheap set of plates made up to carry in the boot.. but saying that they've been on for over 2 years and never been tugged and ANPR cameras read them fine
Obviously up to you but if your plates are legal then leave them on. I know it's time/effort if you get stopped, but I don't carry spares in my car as I know i'm 100%. If I am given a FPN, I contest it, have my day in court and win - that's if it even gets that far!