They're simply not allowed to have ads (or direct subscriber fees) in front of the license-paid British public. The same content may be available elsewhere with

They're simply not allowed to have ads (or direct subscriber fees) in front of the license-paid British public. The same content may be available elsewhere with adverts or fees (and that does provide a substantial chunk of their income), but those extra money-making bits are only allowed offshore. The British public have already paid for what they get, so double-dipping is not allowed. (And unlike some other "public" broadcasters, your programming isn't regularly preempted for fundraising begathons featuring programming that almost none of the regular viewers like.)