"If you want to practise your driving with an accompanying driver, the accompanying driver must be over the age of 21 and must have held (and still hold) a full licence, in the relevant vehicle category, for three years."
http://www.direct.gov...riveOrRide/DG_4022661
No mention of needing insurance to accompany a learner but it makes sense. If the learner gets into a position they feel uncomfortable in and the accompanying driver has to take over then he would have to be insured.
If the accompanying driver has his own car covered fully comp. he would probably be covered to drive the car on his own policy. Check the policy to be sure.