Apart from the Collins Gem. - which are great - you could go for a "Flora". It's a book (doh) that leads you step by step through the identification process of a plant / tree.
On very populair / well known plants it normaly says that you can eat some particular parts (strawberry for example: the fruit). Though not on the anonymous plants. That's something you have to find out yourself (for the moment
). So that could be a drawback for not purchasing a "Flora".
Other arguments for not using a "Flora" would be the time it takes to ID a plant, and it could be a bit difficult (well, not that difficult - I can use too) due to the botanical terms that are used in it. (Good "Flora's" have pictures or some explanation about the terms that are used).
On the plus side it gives you an almost 100% positive ID on a plant - it really depends on your skill / dedication to follow the book closely at every single step towards the identification. If you decide to take some short cutts, the identification isn't really save (though if you use it a lot, you'll recognise plants specific plants by their own characteristics and can skipp the ID process).
One specific for the British Isles:
http://www.amazon.co.uk/exec/obidos/ASIN/0521653150/ref=sib_rdr_dp/202-2293446-2672601
I know there's another one, but can't find the link in my Favourites at the moment ...
Have a look. There are some pages of the "Flora" online, so you have an idea of what I mean.
Have a look at your local bookshop if you can find one and flick trough it. Remember you can always make notes in a seperate notebook about the uses of some plants you want to use, or - if you do it in an organised fashion in the "Flora" itself. This way you can ID every single plant in the UK, and know it's use.
BTW - keep a close eye on the forum in a month or 2
Hope this helps!
