They flower now (I posted a photo of one of my patches a couple of days ago). I have both the pink and white types - the white flower at the end of August and the pink in September. Any well draining soil should work, but will have to be kept frost free. They will flower and then put up leaves and go dormant in the summer. It is used in roadside landscaping here, so they get no water in the summer. But mid August, you should put them outside to get rain water and start the cycle. During the winter they will need sun, and unless you have a conservatory, that's why few people grow them where it is cold/freezing. They will also take a lot of crowding. Use a large pot because the roots are big and thick. A little fertilizer after they flower. If you collect the seed, they should be started as soon as they are ripe (it's how I am increasing my white ones).

Thanks for you answer. I think I am going to have a problem keeping them here in England.