It may also be the case that some or all of their content is licensed for a non-UK audience, meaning that they would have higher costs if they wanted to allow U

It may also be the case that some or all of their content is licensed for a non-UK audience, meaning that they would have higher costs if they wanted to allow UK viewers, a cost which they might not recoup from UK advertisement viewers. That said, based on the limited information we have, I'm totally with you that it's a ridiculous situation. Maybe they have a somewhat valid reason, such as the one I suggested above, but I do doubt it.