That levying money
If the loans were raised by No10, as alleged. Was tax paid on the interest? The party treasurer should know this (in Law). The requirements of a Treasurer is to ensure that the organisation complies with its governing document, company law and any other relevant legislation or regulations.
If it didn't go through the Labour Party books, then surely it was the Government that was raising the money
Bill of Rights 1689
That levying money for or to the use of the crown (Government), by pretence of prerogative, without grant of parliament, for longer time, or in other manner than the same is or shall be granted, is illegal.