Bedfont Lakes Country Park, near Feltham in Hounslow. It's where I work.
We have a range of habitats home to over 300 plant species, 150+ birds, 240+ moths and that's just for starters. We have the largest known breeding site for Goat Moths in London (there's only one other apparently), wintering Bittern and Smew, and Cetti's Warbler, 40 known UKBAP priority species, the largest natural reedbed in western London with 50-60 pairs of Reed Warbler, and we are the only site ever to have been awarded the London in Bloom Wildflower and Environment Trophy twice. Come and see what you've been missing.
Cheers
Adam