Afaik DDoS is illegal in some countrys. Isnt it even illegal to own the tools in the UK?
Anyway, i see no issue with a service like that.
Well considering this does the same seen as your web browser, it's not illegal.
UK is just a stupid nation with a lot of even stupider laws.
Real countries have a lot more important things to worry about such as credit card fraud, tracking terrorists, etc.
Even then, I wouldn't expect anyone gets in trouble for DDoS over there.