You can find the odd mix here and there... Brownies, Carrot Cake, Devils Food.... some UK ones as well. Just no where near the huge amount of US brands, nor UK brands for that matter, that you would find in the USA. The selection is limited to 15 to 20 over various types of mixes not the 50 cake mixes you see in the USA.
Like Otterpop says you can learn to make things from scratch.