Errr... have you actually watched the full speech (available
here) or have you just naively believed a tweet that takes a couple of seconds of it completely out of context? If you take the trouble to watch it you'll realise that Boris was not in any way comparing the suffering of Ukraine with Brexit (if he had been doing that, it would've arguably been offensive, but he wasn't). He was making a wider point that people across the World will always choose freedom if given the choice, and using a couple of different examples to demonstrate that point.
I'm afraid this looks to me like the usual left-wing thing of gratuitously looking for ways to get offended by things that their political opponents say. No story here. Time to move on.
(Ironically, in taking offence at something taken out of context, you also completely passed over the one thing that
is arguably offensive [*]: The Tweet you linked to describing a political opponent as 'this loathsome man'. Personally I have standards in debate that tend to mean I prefer not to dehumanize political opponents by describing them in that kind of way. Maybe your standards differ?)
[*] In a debating, domestic political context of course. I'm not sure anything within the UK is even 0.1% as offensive as what Putin is doing in Ukraine.