'Actually no there’s not one “truth’, it’s always the truth as we remember it. And memories change.'
Sure, but when you're using your hazy truth to slag off your family and stir the shit, it's not great.
There are passages in the book where Harry speaks of his Dad doing stuff like coming up to say goodnight every night, leaving the landing light on and the bedroom door open because Harry didn't like the dark. These aren't the actions of an awful father, they're the actions of a loving, sensitive father. So perhaps Harry's unreliable memory has forgotten many other instances of kind, loving parenting, hugs etc.
His memory being as true as objective fact is bollocks. 'His truth' vs 'the truth'. He's an unreliable narrator, the issue being that he's an unreliable narrator slagging off his family and the constitutional monarchy.