An FAA third-class medical is sufficient for the validation mentioned in post 4.
Art 8 para 3 in Commission Delegated Regulation (EU) 2020/723—
UK version at: https://www.caa.co.uk/uk-regulations...gm-cs/aircrew/
"[...] the CAA may validate a licence which is equivalent to one of
those referred to in paragraph 2 [a PPL, Balloon Pilot Licence, or a
Sailplane Pilot Licence] and issued in compliance with the
requirements of Annex 1 to the Chicago Convention by a third country
for a maximum of 28 days per calendar year for specific non-commercial
tasks, provided that the applicant complies with all of the following
requirements:
(a) holds an appropriate licence and medical certificate and
associated ratings or qualifications issued in accordance with Annex 1
to the Chicago Convention;
(b) has completed at least one acclimatisation flight with a qualified
instructor prior to carrying out the specific tasks of limited
duration."