The trip can't have been that last minute, they would have had to organise tests to go and land in Dubai with a negative text within 72 hours of the test being done. You'd also have to factor in organising a test and it was quite busy before Christmas. If they flew Boxing Day they would have had to have been tested the 23rd/24th so did they have Christmas with just the three of them then? Because their tests would have been worthless if they met up with Fleur's family after doing the tests. I don't think she would have stayed away from her family for Christmas day, she's shown she doesn't care about rules or morals.
I have a lot friends who either got stuck in the UK or couldn't fly home to the UK because countries were banning flights to and from the UK. That was a week before Christmas. Fleur and Mike knew well what was going on. I don't think they would have booked flights when all that started. It was most likely booked before and they just decided "duck it, unless Boris Johnson shows up at the Hall and personally tells us to not go it's totally fine for us to jet of on a luxury vacation, no one will ever know"
As so many people keep saying, it doesn't matter what Tier they were in or going into they know that traveling is morally wrong. The most shocking part of all this is that Fleur went to so much bother to lie and then thought she'd get away with it. The whole thing was planned out and it was not last minute imo. And I do still think that she timed it over her usual Christmas break so no one would be suspicious.