The reason that it is banned is because research has shown it to have two rather nasty side-effects. These are
1) it produces effects in the male reproductive system that could affect fertility
2) a component part of Stevia, steviol, is genotoxic (i.e. it causes mutations in DNA).
Food Standards Agency response states:
"Many sweeteners are not allowed to be imported or sold in the UK. This is because sweeteners and other food additives are tightly regulated within the EU and may only be used once their safety has been rigorously assessed. An example of a sweetener that is not allowed to be imported or sold in the UK is Stevioside, which is also known as Stevia rebaudiana Bertoni . This does not have clearance to be marketed in the UK as it is a high intensity sweetener, 250-300 times sweeter than sucrose."
Please be aware of the possible side-effects if you are tempted to try it. There are many much safer alternatives.