Very much depends on the copper and his/her mood at the time. Most are nice people and will respond well to a good attitude, however like every area in society there are some bad eggs.
A few years back I was pulled for displaying no tax disc (it had literally fallen into the footwell) I stuck it back on and said I would get a new holder asap and was perfectly polite, but he was a complete A Hole (bad day I don't know), but fine me £30... WTF, I wasn't too impressed and made him realise it to, at which point he said if I didn't shut up he would do me for dangerous driving... WTF!! Absolutely no evidence for that at all, but I did shut up, I was fuming by that point and was going to slam his passenger door as I left... but I just left it wide open instead, which made him have to get out and close it; lol, childish I know, but made me feel a little better.
By carrying the proper plates in the car may just show that you KNOW that you are breaking the law but doing it anyway, and could be worse than not having them at all... is it worth the risk, probably not mate.
However, you could state that you've just bought them to replace the incorrectly spaced ones...