I know you've been to London 4x already but have you been that time of year? It's a different London than say April or the summer. You'll have Christmas markets, concerts, pantomimes, shows... Hyde Park and Kew Gardens are magnificent with their lights and decor. Oxford Street's festive. It feels like a different city.
If you have your heart set on day trips though you have Cambridge, Oxford, or Bath. They're better during the warm months than the winter though. Bath's one of my favorite cities in the world - You have to be slow paced. Just live. Ironically much like Paris. Cambridge, you could go punting but dress warmly. All three of those cities you could spend a full day walking around and sightseeing and make it back to London for a late dinner (keep in mind the sun sets around 4:30p if that matters to you).
I'd skip Manchester. There's honestly not much to see or do there. Much rather go to York or the Lake District for a few days.