I just think it might encourage people to think ' fuck it - we have lockdown coming anyway, we might as well all meet up now and have a good Christmas before things get all shit and lonely again.'
Whereas in areas where they don't know if lockdown is coming they might be more likely to think 'well, if we don't all meet up now and infection rates don't increase we might avoid another bloody lockdown.'
(I have no skin in the game. I'm not meeting family over Christmas anyway - just wondering if this has been a big tactical error).