I was looking for a demo of this game. And I came across this site:http://www.sherlockholmes-thegame.com/index.php?rub=shop
As you can see you can get it retail, the disk version, at game.co.uk
I have also seen it on sale at a local gamestop store near me (in Denmark)
In the UK you'll probably pay sales tax and customs tax since it is imported from the US. + sales tax and customs tax on the shipping also.
At least I had to pay this when I bought some adventure games some years ago from the US.