Given that both of the revamps of Jekka's books recently have been done in conjunction with the RHS, and they give her gold medals! Its understandable that shes promoting membership to them. Seems odd she didn't mention the West of England Herb Group though as she's their president.
Re being a member of the RHS, I'm not one personally, I guess membership to them would work if you have an interest in all kinds of plants, but as a self confessed Herbaholic I wouldn't get my fix from the RHS, looking at the difference in price to the RHS and the Herb Society they have a magazine that comes out every month retail price £4.95, so that's probably taking up a chunk of the fee. I guess I prefer to get a mag that is all about herbs which are my passion Having entry to all those gardens only works if you can get there as well, I guess at the end of the day you pays your money and you takes your choice