it doesn't matter who was driving - the van is essentially in your husbands name, so he is responsible.
the fact that he doesn't have a record of who has his van is no-one elses fault but his own.
had you been able to give the drivers details then maybe you could have avoided all this, but as you didn't the blame has been placed on you.
if you can find out who was driving, you could perhaps recover the money by wage deductions, but your husband is the one who must deal with the issue legally, as it has now gone further than a speeding ticket, and as you cannot just say you don't know who has driven your vehicle and expect the matter to be dropped.
lots of people complain about lots of speed cameras - this is irrelevant
i suggest pleading guilty, pay up and get your money back from the person driving...perhaps go through job sheets and hours worked etc to work out who it was.
i would also suggest contacting them and asking why they need all that info and why ot is relevant - i suspect its for them to work out fine repayments etc, but this is the only way to be sure.