bookworm
Art 259(3) still does not define a club. From Sched 7:
UK CPL
(5) Subject to paragraph (6), the holder is entitled to fly as pilot in command of an aeroplane of a type or class specified in an instructor’s rating included in the licence on a flight for the purpose of aerial work which consists of instruction or testing in a club environment.
UK BCPL:
(4) Subject to paragraph (5), the holder is entitled to fly as pilot in command of an aeroplane of a type or class specified in an instructor’s rating included in the licence on an aerial work flight which consists of instruction or testing in a club environment.
JAA CPL
(4) Subject to paragraph (5), the holder is entitled to fly as pilot in command of an aeroplane of a type or class specified in any flying instructor’s rating, class rating instructor rating, flight instructor rating or assistant flying instructor’s rating included in the licence on an aerial work flight which consists of instruction or testing in a club environment.
NPPL
(5) The circumstances secondly referred to in paragraph (3) are that the holder flies such an aeroplane for the purposes of aerial work which consists of instruction or testing in a club environment provided that, in the case of instruction, the licence includes a flying instructor’s rating or an assistant flying instructor’s rating.
The phrase "club environment" has in the latest ANO replaced the earlier phrase "in a flying club of which both are members" It doesn't appear in Helicopter licences!
I doubt the CAA are keen to define "club" anytime soon
Quite right. Historically, it goes back to the days when AOCs were first introduced. All flying involving passengers required an AOC, the two exceptions were private flights and flying clubs!