Hi Amanda
I've never seen it available as a plant in the UK, even as a conservatory plant.
It's a native of South Africa, and so i would guess that it isn't hardy enough although it might survive in a polytunnel
I was sent a link by Geoff Barrows for a South African Nursery that will send seeds to the UK
http://www.silverhillseeds.co.za/ByName.asp?SearchText=bruniaYou could use White Hypericum instead, - they are hardy and can be grown outside in the UK
I grow Hypericum Magical White
hope that helps
Claire