If you had said that you were trying to get a tax disc for next month there is a chance someone would have sussed it, people rarely spend money before they have to.
I agree that DVLA are not the most communicative about errors like that: another example is if you try to Sorn a vehicle a day too early: you get a cryptic message that says you cannot do it, no mention of do it tomorrow, and what difference does a few days or weeks make on a SORN? You know if you are not going to use the car.