My understanding is that if airlines stop running flights they may lose their access to a given airport or terminal. So they run flights even if they aren't full or in major demand.
This used to be the case, but the relevant rule has now been suspended.
https://www.euractiv.com/section/aviation/news/eu-council-jo...
Your sibling commenter gambiting (https://news.ycombinator.com/item?id=22782078) points out that, as your link shows, the suspension is an EU, not a UK, decision.
Here is an article stating this explicitly in connection to the "ghost flight" rules: https://www.theguardian.com/business/2020/mar/10/eu-airlines...