It's worth noting that the linked article does offer selection bias as an explanation – suggesting that conservative supporters were more likely to write letters, supporters of other parties less likely to.

Number of complaints seems a pretty poor measure of bias, but it is one that is often used to reinforce the idea of the BBC's impartiality.

Complaints are, like any "flagging" system, being weaponized to move the Overton window. There's plenty of people willing to write in and complain any time the BBC says that global warming is happening, that shouldn't be taken as a need to "correct" that.

> I think that this is because the BBC is naturally biased towards the status-quo.

Yes. And fearful of losing funding.

In the post-trump era, right wing complaints against an organisation aren't necessarily a sign that that organisation is doing a good thing. Complaints from the factions in power in the UK at the moment are usually just a sign that their actions 'arent right enough' rather than not being right wing full stop.

Also, the BBC News wing has separate management, some of whom were appointed during Cameron's time in office. So the drama and comedy produced by the BBC can be independent of its news editing.