The MHRA explains why herbal medicine regulation is important

Herbal medicines can be dangerous if they are contaminated or adultered.  There have also been  occasions when one safe herb has been mistakenly exchanged for a very damaging one with a similar name.

You can read a presentation from the MHRA on herbal regulation (2583 kb) [ppt].  It gives a very clear account of complex policy issues. 

There's also a very detailed description of how the Medicines Act might change on the MHRA website.