to visit a public open space for the purposes of open-air recreation to promote their physical or mental health or emotional wellbeing—
(ii)with one or more members of their household, or
(iii)with one member of another household;
At the time, and when similar restrictions applied later in the year (November lockdown I believe?) for GA this was interpreted as meaning that those with open cockpit aircraft could fly for recreation, since they would be in the open air, and they were undertaking recreation.
It would of course make little sense to say that it's OK to fly if you have an open cockpit but not if you have a canopy or roof, and on that basis closed cockpit aircraft will probably be covered under a reasonable excuse deriving from the same.
Either way it makes little difference - most airfields will remain shut until 29 March as the CAA will probably not change their guidance until then, so the decision will be out of pilots' hands (should that be control columns?)