The Organic Catalogue does number of wildflower seed mixes for different situations. Also try
www.farmdirectonline.co.uk who do grass / wildflower mixes for different situations.
Wildflowers do prefer poor soil mainly because the normally very vigorous grasses, especially new varieties developed for high fertility situations, can't grow to their potential, so the wildflowers get a chance to grow without being choked out. If you can, reduce fertility by cutting and removing grass or by skimming off the top layer of turf and sowing on to compacted bare soil. Don't turn it over as you'll encourage a whole range of other wild flowers that favour broken soil, such as poppies, which thrive in cultivated land.
We have a patch of wildflower meadow in the garden, which I threaten to remove every year because it get so scruffy. We never do - and this year we've been rewarded with a pair of Goldfinches who are feeding on the seedheads. The hens also love it - it must be full of seeds and bugs.