You can bind your torrent client to only use the VPN connection. The default is that if there is a drop on the vpn, most torrent clients use any connection which means it would just use your normal connection and could leak your ip address.

when would I need to switch over to using Lemmy.world?

I can think of 2 reasons:

• feddit.uk is down.
• you want to create a community on Lemmy.world you have to do it with an account on that instance.

Also can you only find certain communities depending on which account your signed into.

so if an instance is defederated with another like beehaw to lemmy.world (though they say this is temporary), then you won't get any updates out of those communities. You can always search the community from another instance in the search tool. If someone else on your instance already did this the community should be accessible via c/communityName@instance.name. That URL will not work unless someone has already searched it and it's indexed. For large instances this will likely not be an issue, but it may be annoying for people on smaller instances.