I have a separate web-site which covers the flora of the North-West Himalaya, sections of which provide detailed information about how to correctly & reliably identify plants from that region of the world - a good deal of the content and advice is applicable to those taking an interest in British wild and naturalised plant species.
Taking & having available on web-sites a larger number of images for every species, showing a wide range of features, greatly increases the likelihood of correct identification. In the past, if books with illustrations were available at all, these tended to be a single photo per species and often of small dimensions. The best way to learn to differentiate between similar-looking species, is to immerse yourself in them - the wonders of digital photography allows us to do this.
1. A fine example of what methodical digital photography has to offer, is to view the 30+ images of 'Himalayan Balasam' (Impatiens glandulifera) I provide. This plant has 'escaped' and naturalised extensively in the UK and many countries in the Northern Hemisphere, in places making a nuisance of itself:
https://sites.google.com/a/shpa.org.uk/fowh/impatiens-1
2. Correctly identifying plants is not as quick and easy a process as you may have thought! There is a new and better way - by taking examining plants more closely: https://sites.google.com/a/shpa.org.uk/fowh/introduction-1
3. Reliability of identification son web-sites is crucial:
4. How not to identify plants:
https://sites.google.com/a/shpa.org.uk/fowh/how-not-to-attempt-to-identify-plants
5. Parts of plants which need to be photographed:
https://sites.google.com/a/shpa.org.uk/fowh/which-parts-of-a-plant-need-to-be-photographed
6. Become a plant 'detective':
https://sites.google.com/a/shpa.org.uk/fowh/be-a-plant-detective
7. Why arrange the web-site in alphabetical order by genus?
https://sites.google.com/a/shpa.org.uk/fowh/why-arrange-this-site-by-genera
8. What sort of camera do you need? A modest, inexpensive model will be good enough!
https://sites.google.com/a/shpa.org.uk/fowh/what-sort-of-camera-should-you-use-for-plant-photography