If you have synchronization with other devices activated, it might be possible that some plugin has changed the url (some nasty plugins added by adware programs change things like your default search engine without asking, so it won’t surprise me if there’s one that changes your favorites to force you to watch some ads before being able to access them). However, safari for iPadOS does not support plugins, as far as I know
Another possibility could be that safari had redirected you to another website to show you the warning as some antivirus like McAffee do. Before panicking, check if more favorites have been affected and if accessing that website from google or writing its url also shows that warning. If it always warn you and the rest of favorites are ok, it might be a problem of the bbc website
Virus don’t usually affect a single url or file, specially when it isn’t one related to a bank, social network or store (the bbc website does not seem a good target for stealing your money)