This was specifically about the cyberattacks and social media attacks, but it's a good explanation of why Putin sees it in his interest to attack the UK.
data.parliament.uk/writtenevidence/committeeevidence.svc/evidencedocument/digital-culture-media-and-sport-committee/fake-news/oral/79824.html
Bill Browder: Well, you should say, what is the objective? Vladimir Putin’s objective is to stay in power and to keep all his money. How does he stay in power and keep all his money? It’s not easy with a bunch of people in his country who are—I mean, any person, whether you are the best leader or the worst leader, if you have been around for 17 years, people start getting tired of you. And in his particular case, they’re getting tired of him, and the economy is not doing well, and people are getting poorer. So what do you do? You go out and you start wars, you create chaos and you create foreign enemies. This is all sort of “Machiavelli Dictator’s Playbook 101” that Putin is after.
So what is his strategy? His strategy is to have foreign enemies that are not at war with him. He can’t be at war; he’s got to have an asymmetric war. He could not survive a real military conflict. This is a perfect asymmetric war: to pretend you’re not doing it, to send in little green men to Crimea and then say, “No, they weren’t ours”, and to do all this stuff where you spend $1.5 million a month sending out messages from the internet research institute in St Petersburg, and things that are plausibly deniable, which have a huge effect, but do not require huge resources to do. He’s become the expert at asymmetric war.
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This strategy is not going to work for him. In the end, we’re all going to anger to such an extent that we will come up with solutions for all the stuff he’s doing. Democracies are slow to anger, but when we do it’s going to be devastating for him.