Peter McCormack appears to live in the UK[1], so establishing jurisdiction is straight forward.

Libel damages vary - £24,000, £40,000, to a few hundred thousand[2] - it depends on the facts of the case (though the loser will also have costs to pay).

[1] https://uk.linkedin.com/in/petermccormack1

[2] https://www.pressgazette.co.uk/food-blogger-jack-monroe-wins... ; https://www.lawgazette.co.uk/law/40000-damages-in-twitter-li... ; https://en.wikipedia.org/wiki/English_defamation_law#Damages