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It's possibly a bit late in the year to get anything like a meadow from seeds. You could try getting some wildflower plugs from someone like Landlife Wildflowers - not cheap at about a £1 a plug. Alternatively, you could get some wildflower turf (but again don't know if it's the right time of year).
Planting some yellow rattle would weaken the grass so you get more luck next year...
Best wildflower seeds for getting the garden looking more like a meadow and less like we just moved in and cut everything down?