Are you getting the ferry in January to Spain or do you intend driving through France to get to Spain, I hope the weather is settled in January, but it's been very unusual and unsettled so far in November, I think you could be right that it's best to head to southern Spain first, but it can be cold in Spain in January.
We have normally gone to Thailand for 3 months in January to escape the winters, so this will be the first time we will miss, although I did say to the Mrs only yesterday that if the weather doesn't start to improve soon here, then perhaps we should go to Thailand for January and set of in the motorhome in February. Have you been to Spain in January, my cousin who lives here said that had some snow last January here.