Policy and guidance on mounting cameras on aircraft.
The CAA have published CAP1369 which details the policy and provides guidance for mounting cameras on aircraft.
If you wish to attach a small camera (such as a GoPro) to a non EASA certified GA aircraft then the attachment needs to be inspected by a Part66 licensed aircraft engineer or via the CAA as a minor modification to the aircraft. To approve any installation the engineer will need to complete a maintenance release checklist and complete the aircraft logbook entry. For aircraft overseen by the British Microlight Aircraft Association or Light Aircraft Association those organisation’s requirements apply.
You should read CAP1369 for full details.