While Sky do state that you must use their router no one has ever been kicked off the service for using a third-party one.
Since Sky wont give out the username/password you need to either extract it directly from the router or generate it from information on the label at the bottom of the router.
With the Netgear and Sagem routers you can easily discover your username/password by entering the relevant information at
this site
.
The Dlink routers are a bit more complicated, but still relatively simple, and requires you to extract the username/password directly from the router by running
this Java app
.
Then it's just a matter of putting your username/password into your Draytek just as with any ISP.
I switched over to Sky LLU just over a month ago and have been using my 2820VN without any problems since (other than the multitude of bugs that the 2820 has had since launch).