I have a library of field guides on a very wide range of natural history and habitat subjects. Lately, all my go-to references are photographic editions. In current books the photos are so good they have replaced my 'illustrated' editions (though I still like them). All the Wild Guides books are excellent.
So, my current recommendations are:
Plants:
Harrop's Wild Flowers - Bloomsbury, Simon Harrop (my go-to book)
but you may need to go to a key for some of the difficult ones such as 'The Wild Flower Key' by Francis Rose and many botanists still use 'Wild Flowers of Britain & Ireland' by Marjorie Blamey
Britain's Ferns - Wild Guides, Merryweather
Trees
Collins Complete Guide to British Trees - Paul Sterry (photo edition)
Birds:
Birds of Britain and Europe - RSPB/DK, Rob Hume
Britain's Birds - Wild Guides, Hume, Still et al (more photos but less detail)
Dragonflies - Wild Guides, Bee et al
Mushrooms - Collins Complete Guide to British Mushrooms and Toadstools, Sterry & Hughes
Or Phillips of course, and for edibles especially Geoff Dann
Pond life - Freshwater Life, Collins Pocket Guide, Greenhalgh & Ovenden
Butterflies & Moths - Collins Complete Guide to British Butterflies & Moths, Sterry, Cleave, & Read
Insects - Britain's Insects - Wild Guides, Brock (the best general insect book, cover 1,653 species, however, any insect book can only cover a fraction of total species)
Mammals - Britain's Mammals - Wild Guides, Couzens et al