A piece of cake in British Slang means that something is very easy to do.
In British Slang the term piece of cake is the same as the American English usage for something being easy to do.
The phrase a piece of cake in British terms and English terms can also mean a literal piece of cake.
For something easy to do the term a piece of cake can be used in a sentence like, "That Job was a piece of cake".
Or that exam was a piece of cake.
For an actual piece of cake you could say "Can I have a piece of cake Please?"
The saying "a piece of cake" means something that's simple to accomplish.
If a school assignment for example, is a piece of cake, it's so easy that you will barely have to think about it.
Other ways to say "it's a piece of cake" include no problem or it's a breeze.