I want to walk off site over unpaid money, but it could get us terminated

When the money stops, walking off site looks like the obvious answer. It is also the step most likely to get you terminated. Take the gangs off without proper notice and your main contractor can call it suspending the works without reasonable cause. Most subcontracts list that as a default: a breach that lets your main contractor start the steps to terminate.

If it does terminate, the money you are owed is not simply paid. It gets argued inside a termination account, usually once someone else has finished your work, with the cost of that set against you.

There is a lawful way to stop, under Section 112 of the Housing Grants, Construction and Regeneration Act 1996. The final date for payment is the last day your main contractor has to pay you. If a sum due is not paid in full by then, you can suspend work. First you must give written notice of your intention to suspend, stating the grounds, at least 7 days before you stop. Some subcontracts also give a right to terminate for non-payment, with their own notices and periods. Both routes depend on the money really being due, on the dates and on the notices already given.

Before anyone leaves site, check what is actually due. Look at your application, the final date for payment, and any Payment Notice or Pay Less Notice saying what your main contractor will pay. If a sum due is unpaid, send written notice of your intention to suspend, stating the grounds. Keep working until the notice period has run. Record how and when the notice went, and keep records showing non-payment was the only reason for stopping. Take any legal doubt to your solicitor first.

Termination Prevention checks whether your subcontract gives you a right to suspend or terminate. It then sets up the notice route your team uses instead of leaving site, so a payment dispute never becomes a default.