I don't know how well it would "translate" to UK usage, but in the US there is an "establishment" reference on herbal medicines that might be of use when you encounter the need to reassure those who question your suggestions about herbals:

The PDR for Herbal Medicines, ©1998 Medical Economics Company, Montvale, New Jersey, ISBN 1-56363-202-6.

It mimics the PDR (Physician's Desk Reference) for prescription drugs that's pretty universally used by establishment practitioners in the US and is put out by the same publishers; and it gives a reasonably authoritative "establishment" view of common herbal remedies, including effectiveness, known side effects, adverse reactions, and interactions between herbals.

Since it covers most herbal remedies that are commercially available in the US, it should include(?) just about anything you're likely to gather in most parts of the world.

Not something that's a "must have" to work with herbals, and at $60 (US) or so list price it's not a casual purchase; but if you encounter "resistance" it could be helpful, and should be available from Barnes/Borders/Amazon etc.

There is increasing awareness by physicians of the use of herbals, and acceptance (sometimes grudging) that people want to use them. This reference is what physicians here are looking at to answer questions about them.

There may, of course, be similar publications more appropriate to your area. The point is that you can probably find an establishment view on herbals to help in responding to those who may present questions about your recommendations - should any such doubters show up at your workshops.

John