"That Ipsos poll suggests that 50% of people think the BBC is impartial, which means that 50% think it’s biased..."
I think you're misreading this particular figure. The 50% stat comes from a question:
"Of all the news sources (TV broadcaster, radio, newspaper, magazine or website), which ONE source are you most likely to turn to if you want impartial news coverage?"
To which 50% of people picked the BBC, and the next highest option was the ITV at 11%. But that doesn't mean that the other 50% thought the BBC was necessarily the least impartial -- just not the *most* impartial. (In that same question, almost all newspapers scored only 1% or 2% support for impartiality.)
On the question where bias in news outlets was considered individually rather than as a competition, the BBC still came out on top, with an average impartiality rating of 6.4 out of 10. I can't say I really know what this number means on its own -- how can any one person meaningfully give impartiality a score? -- but taken as an average the BBC still is judged by those polled to be one of the most reliable and unbiased news outlets out there, way ahead of others such as the Mail, Guardian, Sun etc.
http://downloads.bbc.co.uk/aboutthebbc/insidethebbc/howwework/reports/pdf/bbc_report_trust_and_impartiality_nov_2017.pdf