johnny37 � in response to your statement "committing an offence... invalidates your insurance" �
Being in breach of road traffic law does not invalidate insurance per se. After all most accidents involve some such breach: speeding, driving without due care & attention etc. It is the Ts & Cs of the policy that will dictate reasons to "invalidate your insurance".
The Association of British Insurers (ABI) says that insurance policies will still pay out in full on any third party claims, and in the case of damage to or theft of your own vehicle, payouts may be reduced to reflect the lower market value of a car without an MoT.
Furthermore, if the car has no road tax, the ABI says insurers are still obliged under the Road Traffic Act to pay out for both third party and comprehensive claims. The fact that you are committing an offence by not having tax is irrelevant.
There are policies that insist in the small print that an MoT must be in force otherwise they will refuse to pay other than the mandatory third party claims. It is always worth checking the wording of your policy to understand the full extent of your cover.