I have FINALLY found orange bitters for sale in the UK!
I kept reading about it on cocktail websites and seeing it listed as an ingredient on bar menus, but have been unable to find it to buy in the UK without paying a huge mark-up. My local M&S now stock it and it's lovely!
Just use in place of Angostura bitters, a couple of dashes into whatever you normally drink.
So far, have found it to work well in G&T, V&T, Tom Collins, Old Fashioned (the drink I'd purchased it for specifically), with Prosecco & 10 ml of Chambord, in a dry Martini and in Cosmopolitans.