Ive had a quick look at the mixtures and, as in the majority of such, there are many appropriate and some inappropriate species and an awful lot depends on your particular soil type. So I would check this against the ecological preferences of species in each mix.
Personally, I have found plug plants insufficiently robust for planting into vigorous grassland. I also think they are expensive. You could grow them on a bit I suppose. Better to grow your own. Unless you are looking for uncommon species you could find many of the species you would want for a summer meadow in a flowery road verge e.g. Bird's-foot Trefoil, Knapweeds, Scabious, Oxeye Daisy etc. Some species need a cold treatment but an easy way is to sow seeds (several of a species into a plug with soil based compost) when available and if germination doesn't occur leave in a sheltered spot e.g. under a north facing wall until next spring at least and see what takes. Some species grow from seed very readily. I currently have loads of Common Knapweed, Betony, Bird's-foot Trefoil and Devil's-bit Scabious which I am going to pot on (to 7cm pots) and then transplant into my grassland. The whole process is fun
and your seed will be native. You could buy seed to sow as individual species from say Emorsgate or Johnsons especially for common species that may be less easy to find in an easy place for collecting. I'm lucky because I know of several very good verges near me. You should have the landowners permission I suppose, but I doubt anyone will mind about a roadverge (unless it is a protected one!) and you will only need a few seed heads. You could also see if you can find some Yellow Rattle to sow into the grass as when this is abundant the grass is held in check.
Seed mixtures will work but you really do need infertile soil to give the less competitive wildflowers a chance. I have had good success on colliery spoil and subsoil based habitat creation projects. Only the most robust species are likely to do well in fertile soil incompetition with vigorous grass.
Have fun.
John