Absolutely standard for there to be no contact before a raid: they're raiding because they believe that you will destroy the documents that they're looking for.
My understanding is that as a prosecuting body the IR is entitled to request a particular enforcement of the law and press for certain penalties - obviously it is down to the magistrate/judge to determine the penalty. As this is a criminal action, you are not dealing with the unpaid tax bill here - at a minimum, if convicted, there would be a fine over and above the tax owed (the tax would be pursued separately through a civil action and not a criminal one).
I am surprised that they are prosecuting without any other contact though. This can only be good news for you, as it means that your dh can demonstrate absolute willingness to co-operate. It may also indicate that they're really after the old firm, but are covering their bases, though it would be unusual to pursue a "criminal" case agianst your dh in those circs - I would have expected them to go down the civil route.
If you're looking for advice, then dh's solicitor should contact your dh's local Inspector of Taxes, writing in with a full disclosure of facts, and copying that to the SCO. At least you will have this down on record. As for the case being in October, these cases get resolved/moved/postponed/settled etc up to 5 minutes before the actual hearing. There's plenty of time for action if you wish to avoid publicity.
Out of interest who's telling you that there is no possibility of a custodial sentence? Dh, his solicitor or the SCO?