Yeah, since beetleweights are still very new, there are only a few around. The Robo Challenge events mainly focus on featherweight robots, and since the UK championships are just over a week away, most people attending would probably be giving their attention to the FW class and may not have a beetleweight robot, let alone a fully-working one.
The Robo Challenge guys did offer beetleweights the chance to get some arena time, but it's not always possible to run a competition with so few robots. I also know of a couple of roboteers who were planning on attending with beetleweights but had to pull out due to various reasons. It's unfortunate, especially since Pugster got his beetleweight running in time, but it can take a while for a new weight class to build up a strong presence at events.
I think the only way to avoid something like this happening again is to ask for a sign-up list before the event to see who's competing, and to ask the organisers nearer the time if there are still plans to run a specific robot type. But other than that, it's difficult to tell how much competition there will be.