It’s illegal to drive a vehicle on a road or in a public place without at least 3rd party insurance.
Even if the vehicle itself is insured, if you’re not correctly insured to drive it you could get penalised.
Penalties for uninsured drivers:
The police could give you a fixed penalty of £300 and 6 penalty points if you’re caught driving a vehicle you’re not insured to drive.
If the case goes to court you could get:
an unlimited fine
disqualified from driving
The police also have the power to seize, and in some cases, destroy the vehicle that’s being driven uninsured.