LONDON, Jan 20 (Reuters) - Six in 10 people in Britain now have an unfavourable opinion of Prime Minister Boris Johnson following revelations of parties held at his residence during COVID-19 lockdowns, pollster Ipsos Mori said on Thursday. Ipsos also said that 57% believe Johnson is a bad prime minister, up 6 percentage points since last week. Finance minister Rishi Sunak was the only senior minister to get a net positive score when respondents were asked.