There is a list of protected plants, lichens, mosses, worts etc here
http://www.rootschat.com/links/01nks/It is illegal to pick, uproot or destroy any of the plants on the list.
The wording of Section 13 (which covers England & Wales and Scotland) of the Wildlife & Countryside Act may be found here
http://www.rootschat.com/links/01nkt/ Note 13 (2) covers all wild plants which in essence means
you cannot remove any plant from a gravestone.
"13 Protection of wild plants.
(1)Subject to the provisions of this Part, if any person—
(a)intentionally picks, uproots or destroys any wild plant included in Schedule 8; or
(b)not being an authorised person, intentionally uproots any wild plant not included in that Schedule,
he shall be guilty of an offence."
The above is the wording for England and Wales but is repeated for Scotland.
Cheers
Guy