I disagree with
homer75 &
Ethel. The various Road Traffic Acts (1988, 1991, 2000 etc.) state specifically "
Users of motor vehicles to be insured or secured against third-party risks". To comply with the law either the user is the source of this cover or the vehicle; it is not a necessity that both are.
http://www.opsi.gov.uk/acts/acts1988/Ukpga_198 80052_en_7.htm#mdiv143
http://www.opsi.gov.uk/SI/si2000/20000726.htm
Consider this situation...
The registered keeper of a vehicle will be out of the UK for 3 weeks and his insurance policy for that car is due to expire at the start of this absence. He decides not to renew the policy until his return and so keeps his car "off road" (i.e. not on a road or other public place) but does not declare SORN. Thus the car is registered with the DVLA, taxed, has an MOT but no insurance "in its own right".
Whilst the keeper is away he gives permission for his friend, who has third party cover for "any other vehicle" as part of his own insurance policy, to use the car if needed.
Provided the friend adheres to the Ts & Cs of his own policy (e.g. he may be limited to "emergency use only") then all legal obligations have been satisfied and the car may be driven on the road (or other public place).