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Not a cabbage, Chelidonium majus is a member of the Poppy family (Papaveraceae)
That would explain why I couldn't work out what kind of cabbage it was!
I made the same mistake the first time I saw Greater Celandine (long time ago). Looking at an old copy of Francis Rose - the key feature seems to be stamens as many as or up to 2x number of petals - cabbages, stamens far more than no of petals poppies - hope this helps
The bright yellow sap produced when a stem is broken is also distinctive.
Lat/Lng: 51.47546, -2.57501
OS grid ref: ST601753