Sorry Meeb, you're wrong. There are three categories concerned - a flight may be Private, Aerial Work or Public Transport. The ANO makes no mention of 'Commercial'. Flying instruction is aerial work as is airborne photography, pipelining, para-dropping etc. if the pilot in command receives 'valuable consideration'. It is Public Transport which is subject to CAP 371, not flying instruction, wherever given. That, as Rolling Circle says, is subject to the ANO although the school may impose tighter restrictions.
The appropriate bit of the ANO is Article 65 and reads:
Flight times - responsibilities of flight crew
65 (1) Subject to paragraph (2), a person shall not act as a member of the flight crew of an aircraft
registered in the United Kingdom if at the beginning of the flight the aggregate of all his
previous flight times:
(a) during the period of 28 consecutive days expiring at the end of the day on which the
flight begins exceeds 100 hours; or
(b) during the period of twelve months expiring at the end of the previous month
exceeds 900 hours.
(2) This article shall not apply to a flight made:
(a) in an aircraft of which the maximum total weight authorised does not exceed 1600 kg
and which is not flying for the purpose of public transport or aerial work; or
(b) in an aircraft not flying for the purpose of public transport nor operated by an air
transport undertaking, if at the time when the flight begins the aggregate of all the
flight times of the aforesaid person since he was last medically examined and found fit
by a person approved by the Authority for the purpose of article 22(8) does not
exceed 25 hours.
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Since flying instruction is aerial work, para 2(a) doesn't apply and one is subject to para (1) i.e. no more than 100hrs/28 days and 900hrs/year. There are no limits on length of duty day, amount of flying instruction/day etc.
QED