You do need a book with a "key" and you need to be willing to learn how to use the key and to develop the patience to use it. But hey! we're bushcrafters right? We know all about patience.
Keys keep you from being vague about what features are present on the plant. If you use it, it forces you to stay on track.
Now, the problem with keys is that they nearly all rely on technical terms and if you don't know the terms it's quite frustrating.
So for learning plants in the UK, I recommend Reader's Digest Field Guide to the Wild Flowers of Britain. ISBN 0 276 00217 2. This has a great key that uses plant features that any of us can recognise. It has drawings rather than photos but the drawings are excellent.
I tried several field guides with keys - Collins, for example - but Reader's Digest took me from knowing nothing to being able to identify many plants.
Starting to learn them changed how I saw the world around me. For the first time I recognised that every life form has its place, that each is constrained by environment, and that each has its times. By seeing how what the times for different plants changed in different places, you could see how the climate was different.
I also became much more observant. I came to realise that I only ever saw Cuckoo Flower (AKA Ladies' Smock AKA Cardamine pratensis) in soil that was ever so slightly wet compared to the other soils around, suggesting that perhaps there was clay underneath. Once I walked along the beach at Pevensey (E. Sussex) and pointed out that the sea beet at our feet was edible. My friend commented that they hadn't even noticed there was a plant in front of us. I knew that I had never noticed them before - not until I started learning their names.
Having begun to learn plants before going on a bushcraft course was a huge help when I did go on courses. On one course, we were supposed to find eight edible plants as a test at the end. I found 12. On another advanced course, a chap who was a real expert in fire and tool making, turned the course into a nighmare with his fear of learning plants. We were starving, thus irritable, thus not getting stuff done. I once noted our instructor pointing out ragwort (Oxford ragwort and poisonous) as one of the edible mustards!
It's also really, really useful to try cooking with them. Preparing, cooking and eating a plant really involves you in how that plant is put together. You make lots of cooking mistakes (well, at least I do!). It's quite off-putting. But you learn which techniques work and which don't.
It's great to see bushcraft schools offering cooking courses now. When I did a couple of bushcraft courses in 1991, no-one taught how to prepare food, not even on advanced courses.
Hope this advice helps!