The CAA confirmed that 'previously' simply means that a pilot intending to fly using a PMD must not do so unless he/she has submitted a declaration beforehand.
It's just clumsy legal-weasel wording which has caused the confusion.
It seems very inconsistent that you can fly an RV or other annex 2 aircraft with an EASA PPL / LAPL and medical declaration but not a Cessna 150
1. An RV is an Annex 1(c) aeroplane.
2. G-reg EASA aircraft such as the Cessna 150 may currently be flown by pilots holding Part-FCL licences (under VFR within UK airspace) using a PMD until Apr 2020.