I don't think its illegal to not have it in the plane
Provided that the flight is intended to begin and end at the same aerodrome and does not include passage over the territory of any country other than the United Kingdom, that is true so long as it is kept at the aerodrome at which the flight began. If, however, you intend to land at another aerodrome, your licence, as well as the other mandatory documents listed in Schedule 9 must be carried in the aircraft. But, of course you knew that, didn't you?