Something that includes GNUstep? Perhaps.
But we don't really have annual GNUstep-only events in general. In fact, w
e've mainly had one off events such as the one in Cambridge 2013 and Dublin 2015. And in Dublin 2015, which was also '20th anniversary' meeting, the number of people attending was very low (essentially the core of the core of developers).
If there is more interest, I'm sure we could come up with something. Probably not specifically for UK, but in general.
If you're interested in meeting, chatting and hacking, a few developers should be at FOSDEM. If people want to voice their interest in traveling to a GNUstep-focused event, this thread is a good chance to voice your interest...