Someone has realtime access to Royal Mail's Sorting office monitoring your post

Here in the UK, someone has real time access to the Royal Mails sorting offices computer systems that works out who sent the post based on the delivery round the mail was picked up from and who the post is being sent to. Just another example of Secret Big Data at work, keeping an eye on the population in the modern world so they can interfere and disrupt when they like.

Any guesses who the entity is that has this realtime access to your mail whilst in the postal system?