If I'm registered on a particular instance, say feddit.uk and I want to create a new community but it doesn't particularly fit on the instance on which I'm registered, am I able to create a community on a separate instance, say lemmy.world or do I have to first register an account on lemmy.world?

Yes, you can only create a community on your home instance. You can moderate an existing community on another instance.

You can't create communities on other instances from your instance, but what you can do is you can create an account on that instance, create the community and transfer ownership to your current account or you can ask somebody nice on that instance to create the community and transfer the ownership to your account.

What community are you thinking of creating? Personally I would recommend making it on your home instance, unless there is a strong reason not to do that.

We are a Canada focused instance but we have lots of other general purpose communities. In fact, this is one of them 😄

make sure there isn't already a community for what you want to create https://lemmyverse.net/communities

I thought perhaps I might start a sister community to r/GrotRevolushun, but I suppose Warhammer is British enough. I'd be a pretty rubbish Gobbo without some kind of revolushun going on!