ANO here
http://www.caa.co.uk/docs/33/cap393.pdf Article 126
Aerodromes - public transport of passengers and instruction in flying
126 (1) An aircraft to which this paragraph applies shall not take off or land at a place in the United Kingdom other than:
(a) an aerodrome licensed under this Order for the take-off and landing of such aircraft; or
(b) a Government aerodrome notified as available for the take-off and landing of such aircraft, or in respect of which the person in charge of the aerodrome has given his permission for the particular aircraft to take off or land as the case may be; and in accordance with any conditions subject to which the aerodrome may have been licensed or notified, or subject to which such permission may have been given.
(2) Subject to paragraph (4), paragraph (1) applies to:
(a) any aeroplane of which the maximum total weight authorised exceeds 2730 kg flying on a flight:
(i) for the purpose of the public transport of passengers;
(ii) for the purpose of instruction in flying given to any person for the purpose
of becoming qualified for the grant of a pilot's licence or the inclusion of an
aircraft rating, a night rating or a night qualification in a licence; or
(iii) for the purpose of carrying out flying tests in respect of the grant of a pilot's licence or the inclusion of an aircraft rating or a night rating in a licence;
Microlight Exemption here:
http://www.caa.co.uk/docs/33/ors4_741.pdf
No need for a Licensed aerodrome for IMC training, IR training or FI training because the person being trained is a licensed pilot capable of making their own decisions regarding the safety of the aerodrome. This "strange" regulation is there to prevent unscrupulous schools endangering students by training from a roundabout in the middle of the A1. Microlights and SLMGs have an exemption on the grounds of mass and the limited amount of fuel they carry.