I can only hope this is so they can gain as much information as possible in order to be able to sabotage it, otherwise it just doesn’t make sense. Iran hates Britain almost as much as it hates the US and Israel on principle because of the way we supported the Shah and sponsored a joint MI6/CIA coup to bring him to power in 1953. That said, that would make the decision mystifying from an Iranian point of view, inviting people from the ‘cunning colonial fox’ to spy on their reactors.
UK are merely a deputy assistant Satan..