It might not be illegal, but it goes against the government advice which is designed to keep everyone safe. Fleur, as an influencer, who is somewhat in the public eye, and as someone who people looked to for comfort-viewing in the pandemic with her lockdown vlogs, is more at risk of public scrutiny and criticism if she were to go against this guidance, which she has done, and that is exactly what has happened.
I am a teacher, and we received an email at the start of lockdown from senior management saying that the media will jump on us for breaking any covid rules and just by being in this profession we are held at a higher standard. Am I going to be like 'well it's legal soooo...' and book a holiday? No. I might receive more criticism than others if I did break guidelines, but that's the job I signed up for so I would expect that and understand why. Just how Fleur should have expected, that being in her job where she is somewhat of a public figure, that if she chooses to blatantly ignore guidelines, that there will be backlash.
What some people aren't getting is that it is actually perfectly normal for some people to be held at higher standards than others when it comes to this stuff - if your neighbour who is on furlough from an office job went on holiday you might not agree and complain about it to your family, but you'll be more shocked if it was a doctor, a teacher, a politician, or someone who has a lot of influence over others (Fleur). Yes, some may argue it's unfair that she's held at a higher standard than a lot of regular people (including people in her comments who were like 'I've been on holiday too') but that is a downside that comes with her job. You can't pick and choose what you influence as an influencer, she can't make $$$ off those affiliate links and sponsorships because of her large audience but then expect her large audience not to have an opinion when she blatantly disregards government advice in a pandemic.