Ill suggest Italy - but it will be cold this time of the year(any place in europe for that matter), spain is better in summers..Even I had been to Italy for a week around dec time but it was cold , nevertheless the trip was breathtaking..
My travelogue London ->>Milan -> Venice(Astounding place - must visit!!!) -> Maranello(oh yes mecca of Ferrari enthusiasts) -> Pisa-> Rome->>London
Sorry we travelled on our own so all bookings done directly, so cant recommend any tour packages
Or you could do a week of switzerland ( just a keep a check if any place/peaks are closed for maintenance)