John Chambers has a large range of native British seeds sold separately and in various mixes for several soil situations. There is a smaller range of native British plants and bulbs available. There are also a few books available through the catalogue. I don't know if they ship outside of the UK.
Probably the most comprehensive catalogue for native British seeds, I enjoy reading the catalogue and have had good germination rates from the seeds.
Well worth a look for anyone who is serious about native British wild flowers.