https://www.caa.co.uk/General-Aviati...rivate-pilots/
Non-EASA aircraft – to fly non-EASA aircraft (for example amateur built or microlight aircraft) you can use the same medical certificates that you would for EASA aircraft. Alternatively you can declare your medical fitness to the CAA (subject to certain conditions) by making a Pilot Medical Declaration. This Declaration is will only be valid for non-EASA aircraft and for use within UK airspace.
Self-declaring your medical fitness using the Pilot Medical Declaration
A medical declaration (from 25 August 2016) is an affirmation of your medical 'fitness to fly' and may be used to exercise the privileges of a:
EU Part-FCL Private Pilot Licence (PPL):
National PPL (NPPL);
UK PPL; and
A UK Commercial Pilot Licence (Balloons), non-public transport operations only.
It is valid for flying with up to three passengers on board and in aircraft up to 5700 kg MTOW. The privileges of an Instrument Meteorological Conditions (IMC) or Night Rating (assuming colour safety has previously been checked by an AME) may be exercised on non-EASA aircraft, but not a full Instrument Rating (IR). Subject to certain exemptions from the EASA Aircrew Regulation, the declaration is only valid for non-EASA aircraft and is not automatically valid outside of the UK since it is not an internationally-recognised medical standard, unless permission has been granted from the foreign state you are flying in.
The standard you must meet for the medical declaration is the DVLA Group 1 Ordinary Driving Licence (ODL) and you must not suffer or have suffered from a list of specified 'medical conditions'.