You can buy them. Lucky Charm is not outlawed. They are in special stores in Britain. Expensive, I heard. Basically, General Mills made a business decision to stop selling Lucky Charms in Britain. There are two on going thoughts on this.
First one is that the Leprechaun on Lucky Charms may have offended some people and General Mills decides the Leprechaun is too important for sale and it is better to lose the sale from Britain alone, then everywhere else. This theory is supported by the fact that the green clover marshmallow was previously removed due to sensitive issues.
The second theory is that the British government has deemed Lucky Charms as unhealthy cereal due to high sugar, and therefore General Mills has to pay a premium or tax to sell them in Britain and consequently General Mills decides it is more profitable to not sell them in Britain.
Again, Lucky Charm is not outlawed as you can buy them. The problem is that General Mills stops selling to Britain directly.
You can get them easily on the internet, but they are twice as expensive as in the US, if not more.
P.S.: If you really want to have some Lucky Charms, make sure you get a big thick black marker and cross out the leprechaun on the box before consuming your cereals. You have to show your British pride, you know. Don't let us Americans make fun of your leprechaun.