Darn, I thought I’d chip in and say cx for sure, as they’re superb fun, fast and keep you fit (believe you are in the Chilterns too, and they are very good here, especially in the winter). Thought i’d persuaded myself I didn’t need a fatty, but after reading the above I also find I’m in the n+1 position still.
Cx also replaces the need for a road bike for me.