There is a difference between a licence issued in accordance with Part-FCL and a licence issued in accordance with the ANO, as to the use of the PMD.
Holders of a Part-FCL licence, whether this be a LAPL, PPL, CPL, ATPL etc can use the PMD but only in accordance with MED.A.030 and/or the exemption (ORS4 No. 1597).
MED.A.030 says
(c) When exercising the privileges of a:
(1) light aircraft pilot licence (LAPL), a balloon pilot licence (BPL) issued in accordance with Annex III (Part-BFCL) to Regulation (EU) 2018/395, or a sailplane pilot licence (SPL) issued in accordance with Annex III (Part-SFCL) to Implementing Regulation (EU) 2018/1976, the pilot shall hold at least a valid LAPL medical certificate or have made a medical declaration (except where the pilot is exercising the privileges of a LAPL in respect of a turbine engine helicopter, in which case the pilot shall hold at least a valid LAPL medical certificate);
(2) private pilot licence (PPL), the pilot shall hold at least a valid class 2 medical certificate (except where the pilot is exercising the privileges of a LAPL in respect of aircraft other than turbine engine helicopters, in which case the pilot shall hold at least a valid class 2 medical certificate or have made a medical declaration).
This, therefore, restricts the holder of a Part-FCL licence to exercising the privileges of the LAPL, when operating on a PMD. The privileges of the LAPL, do not include flight instruction.
The same is true when taking advantage of the exemption, paragraph 5(c) says
(c) Part-FCL LAPL(A) and PPL(A) holders must only exercise the privileges stated in FCL.105.A(a), that is to act as Pilot-In-Command (‘PIC’) on single-engine piston aeroplanes or Touring Motor Gliders (‘TMG’) with a maximum certified take-off mass (MTOM) of 2,000 kg or less, carrying a maximum of 3 passengers, such that there are never more than 4 persons on board the aircraft.
However, as Ontrack have correctly stated here if a pilot also holds a PPL, CPL or ATPL issued in accordance with the ANO (referred to as a UK licence), then Article 163 of the ANO applies. Article 163 sets out the PMD, in addition it sets out the disqualifying medical conditions applicable for a pilot if the wish to operate an aircraft with a MTOM of at or less than 5700kg. A person under treatment for cancer would meet one of those disqualifying medical conditions.
This brings us back to the exemption, (ORS 4 No 1597), this removes most of the disqualifying medical conditions, with the exception of ‘they reasonably believe that they meet the medical requirements for a Group 1 (Car) Licence issued by the Driver and Vehicle Licensing Agency and are not taking medication for any psychiatric illness’, but limits the pilot to flying aircraft with a MTOM of at or less than 2000kg and carrying a maximum of 3 passengers, such that there are never more than 4 persons on board the aircraft.
As such you could have the holder of a PPL/CPL or ATPL issued in accordance with the ANO, with a valid Flight Instructors Certificate conduct flight instruction for the issue of a NPPL, LAPL (until the beginning of October 2025) and PPL, having made a PMD to the at or less than 2000kg criteria. They would be able to fly Part 21 aircraft because Article 3 (3) of the Aircrew Regulation was amended to recognise licences issued in accordance with the ANO.
Given the pilot’s condition if they are considering using the exemption and making a medical declaration, they need to be aware of the information on the DVLA Website in relation to medical conditions. There is specific guidance on people under treatment for cancer:
https://www.gov.uk/cancer-and-driving
Finally, and perhaps most importantly pilots are reminded that the essential requirement of pilot medical fitness remains. Licence holders are reminded of their responsibility in the event of a decrease in their fitness with respect to an illness, medical condition, medical surgery, or treatment that may affect the safe operation of an aircraft. Pilots are encouraged to use assessment tools such as ‘I am safe’, to honestly assess their physical and mental health. CAA has recently published a safety sense leaflet on the subject:
Pilot Health and Performance Safety Sense Leaflet