Welcome aboard, Amanda! It's great to have more UK events starting up!
If you're in need of some ideas or guidance, a great place to start is the 2009 How To Guide. This will be updated for 2010, but in the meantime, the 2009 Guide should give you a good head start. You can download a copy here: http://www.cantstoptheserenity.com/file ... 0Guide.pdf
You will need to get permission to screen Dr Horrible. In 2009, this was organised by the Global Team, and I believe that they will be negotiating for permission to screen it in 2010 also. I'm sure we'll hear more on this from the 2010 Global Team as they progress.
Best of luck! And don't forget that there's lots of support available here, so don't hesitate to ask questions and ask for help