You cannot follow the entire instance as such, rather the individual accounts on the instance - such as @BBCRD, @BBC_News_Labs, @Connected_Studio etc.
Kbin users can subscribe to whatever content is shared from social.bbc on federated instances by subscribing to /d/social.bbc, but I'm not sure how much sense that makes. :)
Edit: In Lemmy you'd find the users by entering for example /u/[email protected], but as @roguetrick pointed out Lemmy is not really made for microblogging.