I still have doubts about whether it would be illegal to open mail once it had been delivered.
kempie posted a link to the actual legislation. The relevent section would be: "A person commits an offence if...he opens a postal packet which he knows or reasonably suspects has been incorrectly delivered to him".
But, the Royal Mail's commitment is to deliver to an address not an individual. So, once the letter is through the right letterbox, it is correctly delivered. No matter who it is addressed to.
And as W~M has mentioned, the perfect excuse is readily available.
As to solicitor's letters, personally I wouldn't take that as proof of any legislation. Years ago, my (now) wife had several from her ex's solicitors which plainly talked nonsense and wishful-thinking. She usually found them very amusing.
Not that I agree with your bf's father's actions in anyway - just saying it might not be illegal.