Testing DOS (Denial Of Service)

If you're looking for a testing service which simulates a denial of service attack then you could try the one identified here:

Parabon, a provider of extreme-scale grid computing, is now offering to public sector organisations and private sector companies a risk-free, no-obligation "red team" test attack to help them assess the reliability of critical network assets in the face of distributed denial of service (DDoS) attacks.

The offer features a grid-based load and performance testing application that can replicate a full-scale DDoS attack. This offer is Parabon’s response to the rising incidence of DDoS attacks against U.S. websites.

It's not clear if this service extends to non US websites (such as UK web-enabled applications and websites) but it could be worth contacting them to find out.