To be honest, I'm not sure how the UK one will compare. It's simply due to different attitudes regarding fandom between here and the US. I went to C3, and you couldn't move for, well basically, geeks, everywhere. It was like being in a mini-heaven Look at a regular convention over here, for example, even the big ones. How many people do you see in costume etc? Apart from the actual costuming groups, not many really. Now look at the big cons in the US, Comic Con, Dragon*Con, the Celebrations, and there are hundreds and hundreds that turn up in costume. I have yet to attend any event in the UK that has the same kind of atmosphere as the Celebration I attended in the US. As such, when C4 was announced both myself and V decided that we would really want to be at the US event, since many of the friends we had made in the US whilst at the last one were going to be going. Another part of what made the US one work so well was that they are open 24/7 pretty much (not the event, but everything around it). Can't really say the same for London unless you're in to clubbing.