It's got me beaten, and the decent wildflower searches online. There can't be many purple stem, yellow flower plants around!
It should have been given a drink or a beer before the photoshoot as the flowers are closed, so an ID isn't easy. Did it come from a soggy end of the Fens?
It looks like one of the buttercup/water lily family Ranunculus... Greater Spearwort?
For now, I'm putting my money on Jerusalem sage (aka Yellow clary - Phlomis fruticosa).
Kosaitai- a type of brassica?
F
Aren't brassicas 4 petalled symmetrical flowers, and looking on the web for kosaitai the flowers are not globular structures. I suspects it's an odd hybrid, but send it to RHS and they'll tell you