Copied directly from the handbook on the site, link:
https://secure.endsleigh.co.uk/HostDocs/PolicyBooklets/MOTOR/2009-09-29.pdf (may not work if not logged in, seems to work across browsers with one logged in (FF) and one not (IE8)).
Footer says Page 6, document page number is page 7:
What is not covered
a) We will not provide cover for the legal liability of any person We insure
under this Motor Policy if there is any other insurance in force that covers
the same liability.
b) We will not cover death of or bodily injury to any person arising out of that
person’s employment, except where it is necessary to meet the requirements
of the Road Traffic Acts.
c) Under this section, We will not cover loss of or damage to Your Car or any
other property belonging to or in the care of any person We cover under this
Motor Policy.
Driving other cars
Cover for driving other cars does not
– apply if You have disposed of the car currently described in Your
Statement of Insurance or it has been damaged beyond economical
repair or it has been stolen and not recovered
– apply in any country outside the United Kingdom
– cover loss of or damage to any car You drive that does not belong to You
– apply to any driver other than you
–
apply unless a valid policy of insurance is in force for the car concerned
Edit: Obviously no Tax or MOT is going to land you in a big mess with the police anyway (unless conviently taking it for an MOT), but insurance wise is explicitly stated.
Edit2: another bit of interest on the same page:
Driving other cars
We will also cover You, in respect of Your legal liability to others, to drive a private car not belonging to You and not hired to You under a hire purchase agreement or leased to You under a leasing agreement, provided You have the owner’s permission to drive it and You are permitted to do so by Your Certificate of Motor Insurance and a valid policy of insurance is in force for the car concerned.