The equipment is rented to you by EE and there are no discounts for not using / returning it - you would cause issues if you return it early as when you either upgrade the equipment or leave it will need to be proven as returned at that point.
As @JimM11 has mentioned in the event of any fault with your service you will need to connect the EE equipment so they can diagnose any issues with the line.
Aaa see… this is the time for a healthy debate. Last time I’ve checked the ISP can check the connection all the way to the ONT. As long as I’m not using their equipment they only have to prove that there is a healthy connection at the ONT. I’m not a big fan of ISP OS on routers and I also believe that it should be the customer’s choice to rent equipment and be charged for it. This is a wake up moment and must be better regulated. The renting of the equipment is built in the monthly subscription.
Checking to the ONT or the socket is only half the picture - yes I am sure they can see if the line is experiencing an issue or not which is great if you have no working service but in the case of intermittency / speed issues/ WI-FI etc that's where it comes into it. As most people typically use the equipment supplied I'm not sure how popular additional charges would be for using an ISPs equipment, given you would like to see an option to remove it if returned etc I would presume it would have to be broken down on the bill to be transparent in the same my mobile flex pay bill shows the difference between my handset and airtime costs
