Did you have to massively change the soil structure to be successful? I have to say the heavy clay we have does put me off. I'm on clay - the area next door was called on the Tithe Map the "Clay Fields". It's a local frost pocket, and the bottom is under several inches of water after heavy rain. I planted a swamp cypress 
I do make compost and spread it, with priority on flower and vegetable/fruit areas, so the wilder bits only get mulch perhaps once every 10 years. The "woodland" at the far end obviously has leaves falling on it, and we put all less easily compostable trimmings down there to rot. It's too dark for much to grow under the canopy - basically ramsons and one of the speedwells, and the dog violets and forget-me-nots, all of which are in spring before the tree canopy has closed.
But it's still basically unimproved in the wilder bits, especially the bits that used to be lawn (which is where a lot of the wildflowers are). All the plants are chosen for that sort of habitat, which is one reason why I've never gone for a standard "meadow" mix. The things doing well in my rather shady "meadow" are greater burnet, meadow cranesbill and betony - so rather taller than most people imagine for a wild flower meadow.
Jack-by-the-hedge was out saviour in March when delivery slots were so difficult - a lot of our lunchtime salads had that to bulk out the bittercress, lambs lettuce, marjoram and chives.
So in summary, don't worry about your soil, just choose plants accordingly and you'll do well.
www.naturescape.co.uk/product/red-campion-plugs/ I bought from naturescape last year, both to supplement my garden, and for the nature park that I help manage. I was very impressed with the quality of their stuff. The plugs were small - they came in containers less than in inch (about 2cm) wide. I potted them into 3 inch pots on arrival in Spring, and by early summer they were sturdy plants that I was confident to let loose - they grew away astonishingly well. By the time I was able to get up to the nature park for planting, the red campion were outgrowing 6 inch pots.