Is it illegal to open someone else's mail in the UK?

I moved out in January but my name is on the mortgage along with my brothers, he never tells me when I have post and doesn't like me going round to the house. Today I found a letter addressed to me that he has opened and not told me about. What can I do? He is already screwing me over with the house so before anyone says do you really want to get him in trouble? Yes. Yes I do.

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