Are you searching on the website or in Jerboa?
The search is a bit funny. If you're on the website and search and if the community hasn't been indexed by your instance, you won't get it in the results. But, if you search with the url https://feddit.uk/c/britishproblems even though it doesn't give results it will go and index the community. A few minutes later it will show up.
I don't know if that works in Jerboa (android app)