Boots have this thing where they only stock certain brands in their larger stores. Brands like Hard Candy, Urban Decay, Benefit, Origins, Too Faced, Karen Millen, Jemma kidd, Ruby and Millie etc. In London there wil be several larger stores that will stock it. I'm not sure where though. I'm sure there's some kind of search you can do on their website which tells you which branch stocks which range.
Urban Decay is also available in department stores. I know for definite select Debenhams stores stock it. I'm not sure what others though, and which branches.
Online, you can buy it from
www.hqhair.com or
www.boots.com. Certain discount makeup websites stock have some products as well.
www.magicmakeup.co.uk were selling a selection of the good karma brushes for £5 or just over last month, even the blusher one. Some of these products are discontinued or they don't tend to get products back in once they sell out. If you can find something that isn't cheap because it's disc and therefore possibly quite old. Then they are a good bargain.
HTH