Is it illegal not to pay a subcontractor?

Because it is not a crime, there is no authority to report it to. What you get instead is quicker than a court case. Start with the notified sum, the sum the notices fix. If your main contractor gave no valid notices, it is the sum you applied for. That comes from section 110B of the Housing Grants, Construction and Regeneration Act 1996. It is due on the final date for payment. Once that date passes unpaid, you can suspend work until you are paid, after giving the notice section 112 and your contract require. The time and reasonable costs of the suspension are recoverable. Interest runs on the late amount, under your contract or the law. And you can refer the dispute to adjudication at any time under section 108. The decision binds both sides until the dispute is finally decided, and the courts enforce it as a matter of routine. Keep that order. Every later step is stronger when the sum itself is not in dispute. Anything beyond the commercial position is for your solicitor.