If you want an instant effect with a mix of wildflowers and grasses similar to our Special General Meadow seed mix, or you are thinking about a green roof, then you might want to look at wildflower, or "meadow" turf.
We explain about the pros and cons of using wildflower turf in our blog. It has a lot going for it when properly prepared and in the right location; it is easy to use and establishes immediately. On the other hand it's expensive and less flexible than using different seed mixes.
We no longer sell it ourselves, but if you are interested we recommend the good folks at Wildflower Turf - do get in touch with them directly.