I'm not convinced it's criminal. As Deetildeath said, it's morally wrong for sure and there have been warning signs that the party ignored. Politically that is difficult for the party to wriggle out of.
Also, I don't think the £600k being spent is the only issue though. I suspect some of the spending is dubious as well - not sure its Trump levels but I think even what it's been spent on and how it's been reported to the Elecotral commission and reported in the Accounts is an issue as well.
As I say, this is my suspicions and gut feel. It could be nonsense on my part.