Yes, that's right, they will mix it for you, or you can buy both from Boots and do it yourself. You'll find the bottles of rosewater on the same shelf as the lavender water.
I used to buy it from the chemistsin a large brown glass bottle for really cheap and it was simply pure rosewater... but 2 years ago, every chemists i went to had stopped selling it for some reason and i can't get it anywhere... but haven't tried for some time as i thought they had stopped making it.
I'll try again today.... (she says with a sheepish shrug...)
You'll have better luck at a small independent chemist. If all else fails, I saw it in Tesco's last week (on the "home baking" section) - it's a lot more expensive there, though!