they don't really look like blackthorn stems, or leaves, too broad. Ladybird is right re the thorns. No thorns, no blackthorn.
have you more photos of the blue one please? one where we can see the leaves clearly.
and an idea of size
I'd do both
Not a love in a mist flower, they have the dark bits in the middle, feathery leaves around, different arrangement of buds and flowers.
spuds are quite pretty
larkspur a strong contender given the leaf description. Leaves are as important as flowers for ID.
One flower per stem for sanguinarias I think, though I haven't seen one lately
there are several different plants there. that main one looks almost paeony-like in some pics
just had a look, monthly open days through the summer now. It was open daily in the past
West Acre gardens and nursery is really great. I have a lot of their plants.
I thought Woottens was open to the public though I've never been and maybe confusing it with somewhere else
should have a rough feel to the leaves and very tough stems