Not residing in the U.K. or being an Octopus customer, I haven't paid close attention to this problem. However, I don't recall reading that any i3 owner outside the U.K. has been suffering this problem. If so, what would make this problem U.K.-specific? There are probably BMW servers just for the U.K., but if this is a BMW server problem, BMW should be able to install server software from another market not suffering this problem. I'm sure some localization would have to be performed, but that shouldn't be a big deal unless this software is really poorly designed and/or written.
Certainly, BMW would not allow any 3rd-party company like Octopus to install its own software on an i3 because of the significant security risks. However, if Octopus somehow installed its own software on an i3 charger module, for example, this software could continue modifying charging schedules even if an i3 owner is no longer an Octopus customer or merely bought a used i3 from an Octopus customer. This seems highly unlikely, but software from various electronic module manufacturers runs on i3's already. Unlike Tesla, BMW didn't write all the software that's running on its EV's.