You shouldn't remove wild plants from the wild, but if you look our for when the plants set seed, I don't think it's so bad to remove a couple of seed heads to dry and sow the following year. You can also try Nicky's Seedshttp://www.nickys-nursery.co.uk/garden-shop/seeds/herbs/
alternatively see if you can organise a 'seed swap' with local people, contact local garden/herb groups. If there isn't one near you think about setting one up?!
I'd second Sarah's suggestion to use Zoe's book and would also add Hedgerow Medicine by Julie Bruton-Seal not as portable but full of brilliant remedie recipes, good clear photos and loads of useful info. Also Food for Free by Richard Mabey.