In reply to goldmember:
The definitive guide is probably The Wildflower Key by Francis Rose, It's technical (maybe too technical) but the illustrations are very good and combined with the descriptions of the plants and their habitats, you can use it without a lot of botanical knowledge, but it IS a biggish book and maybe a bit more professional than you need.
At the other end, the Field Studies Council do an excellent series of fold out laminates detailing common plant species habitat by habitat from peat bogs to playing fields. start with the "what's that flower" pack, perhaps?
http://www.field-studies-council.org/publications/pubs/wildlife-pack-08-wha...
Also lots of good books here
https://www.nhbs.com/
(they are pricier than the like of Amazon, but I'm guessing their tax structure isn't as.... efficient)
Post edited at 11:36