Is buying a cable for a "beIN Sports" device for forty pounds, which broadcasts two channels, forbidden, and what is the expiation for that if it is forbidden?

This action is impermissible due to its infringement upon the rights of channel owners. One must cease this practice and seek forgiveness and pardon from the rights holder. If this is not achieved, then you are obligated to compensate them for the benefits they missed out on and the harm they incurred due to your action, according to the assessment of experts. If you are unable to deliver the monetary value, then you should give it as charity on their behalf to the poor or for the benefit of Muslims.