It's mostly in the Air Navigation Order:
From the Air Navigation Order, 2016:
You need a licence to use a radio in an aircraft
79.—(1) A radio station in an aircraft must not be operated, whether or not the aircraft is in flight, except—
(a) in accordance with the conditions of the licence issued for that station under the law of the country in which the aircraft is registered or the State of the operator; and
(b) by a person duly licensed or otherwise permitted to operate the radio station under that law.
(2) The radio station in an aircraft must not be operated so as to cause interference which impairs the efficiency of aeronautical telecommunications or navigational services.
But not if you're a glider pilot not talking to an ATSU.
139.—(1) A person may act as a flight radiotelephony operator within the United Kingdom, the Channel Islands and the Isle of Man without being the holder of an appropriate licence granted or rendered valid under this Order, if the conditions in paragraph (2) apply.
(2) The conditions referred to in paragraph (1) are that the person is—
(a) the pilot of a balloon or glider and does not communicate by radiotelephony with any air traffic control unit, flight information unit or air/ground communications service unit; or
(b) being trained in an aircraft registered in the United Kingdom to perform duties as a member of the flight crew of an aircraft.
So the key is not what the classification of the frequency is, but that it's a pilot of a glider and they are not communicating with an ATSU (Air Traffic Service Unit). An ATSU is an Air Ground Communications Service or an Airfield Flight Information Service, or an Air Traffic Control unit. So a glider pilot doesn't need a FRTOL to talk on a non-recreational-gliding frequency without such a unit - Gransden Lodge, or HusBos for example.
The relevant CAA policy on this can be found here
They classify recreational stations as a type of 'Operational Control'
"Recreational Aviation comprises associations, clubs, societies and individuals operating gliders, hang gliders, para gliders, paramotors, microlights, parachutes, balloons, gyroplanes and simple single engine aeroplanes (SSEA).
Aeronautical radio stations, comprising fixed, mobile, portable and hand-held radio equipment, are typically established and operated by these clubs, societies and individuals to provide radio communications with aircraft for the exchange of messages related to the particular recreational aviation activity."
They are clear that a radio license is not required for such use, though users need to be 'competent'
"With the exception of providing Information for parachutists a Radio Operator's Certificate of Competence issued by the UK CAA is not required in order to use an aeronautical radio station when providing an OPC communications service; however Article 205 of the ANO is still relevant and places requirements upon operators to be competent. In addition some Recreation Aviation Organisations require members to meet their own competence requirements and reference should be made to the relevant organisation."
If you make any transmissions from a radio on the ground (and not in an aircraft), you'll need a radio license to use your allocated gliding recreational radio frequency (it's more complicated if you are using a non-recreational frequency - talk to me).
You'll need to fill in OfCom's OfW586a form. There's an example here for a gliding club using a recreational frequency. Probably worth checking your draft with me before sending it in.
You will need a license for radios in aircraft. You can find the details at OfCom's site, here. There are two types:
A fixed aircraft radio
A portable radio, which can be used in a number of aircraft.