In message , K
writes

You're allowed to collect 'the four F's' - flowers, fruit, fungi, and
(IIRC) foliage, provided they are for personal use and not for sale,
and provided you have permission to be there (so along a public right
of way is OK, picking mushrooms from a farm field is not, unless you
have the farmer's permission)

Protected species are sufficiently rare for you to be unlikely to come
across them (and most of them seem to be small and inconspicuous).

Bluebells have some specific legislation preventing people taking bulbs
from the wild and selling them.

I'll confess to digging up a root of wild garlic from a nearby country
park to try in my garden. There is a notice on one of the main
paths which says DO NOT PICK WILDFLOWERS , but since I do a couple of
hours a week voluntary conservation work there I claim the right to a
single clump, from an abundance.
We have been removing Rhododendron which had been overpowering certain
areas, and pulling up Indian balsam, which seeks to take over the entire
area. There is also some Japanese Knotweed, the chemical treatment of
which apparently requires a licence, which has to be applied for on each
occasion.

I was surprised when the Warden told me that the paths through the park
were neither ROW nor concessionary paths, but were designated
"unadopted", so I suppose that the notice is a polite request.
--
Gordon H