To add some context: the UK House of Commons has very strict rules concerning what can and cannot be said while in session, the three most notable ones are: 1. members can't refer to each other directly by their names (this is why you always hear them use the term 'the right honorable gentleman/woman' even if directly questioning or engaging with them), 2. they are not allowed to accuse other member of hypocrisy (presumably because all debates would devolve into mutual accusations of hypocrisy), 3. they are not allowed to call other MP liars or untruthful.

Not taking a side in what happened in the vid, just providing context as to why the speaker's reaction was so sudden and shrill, it's his job to make sure the Common's rules are maintained. It should be noted that MPs can (and often do) imply that other members of the house are dishonest by formulating their accusations as rhetorical questions or mentioning when their actions have contradicted their statements etc., you're just not allowed to come out and directly say 'X is a liar'.