There are a number of excellent websites providing information about bugs.
British Bugs: A comprehensive guide to the bugs of the United Kingdom with an excellent set of gallery pages and information on identification.
Koleopterie.de: A German site, but with a large number of images covering a wide range of UK species.
Miridae.dk: A Danish site primarily covering the Miridae. Covers many UK species, including information on host plants and phenology. Has particularly good coverage of nymphs.
Unfortunately there is no up to date field guide covering the majority of the UK's Heteroptera, with Southwood & Leston's 1959 publication, Land and Water Bugs of the British Isles, being the most recent. General insect guides are generally quite poor at covering the bugs, with Paul Brock's A Comprehensive Guide to Insects of Britain & Ireland the best.