The Post Office has a number of special powers, grandfathered from when it was the monopoly communications provider in the UK (it ran both telephone and post). it was effectively an arm of government, that worked hand-in-hand with the security services. Their Investigation Division was responsible for steaming open letters and tapping phone-calls.

I imagine much has changed; but I'm quite sure that the PO is still not an ordinary business, like a courier service. They are an arm of government.