All British travel insurance policies specifically exclude travel in any aircraft other than scheduled passenger services by a recognised airline. Handy, that, for those of us who want to aviate while on holiday.
I think it is more along the lines of "other than as a fare paying passenger". Therefore it doesn't exclude stuff like Grand Canyon helicopter flights or air taxi operations.
Anyway, Traffords, about £100pa for an annual travel insurance policy which also covers flying in private aeroplanes as a passenger or pilot.