In my opinion, thinking about getting people on an instance is probably the wrong way to go about it. The miracle of federation means it doesn't really matter who is on which instance anyway, and having more people on one instance does bring drawbacks (extra workload for the admin, extra muscle needed for the server, etc.).
What's more important is managing to promote involvement in specific communities on this instance. Looking at Reddit, the really big UK centric subs were the main United Kingdom one, UK Politics, Casual UK, and the various location-specific ones. Trying to make sure that Feddit UK's equivalents to these communities are popular and become the main Lemmy successors to these subs (as opposed to similar communities on the giant Lemmy instances) is the challenge.
Not that I have any answers on "how", though. Keep posting content, participating in discussions, and upvoting. Also, if you have an account on one of the big instances too, make sure to subscribe to your Feddit UK communities on those accounts to make sure those servers are pulling the data accross, improving visibility for other users on those instances.