Have you tried to book via BAEC? Should be the same availability and surcharges. (unless you are trying to book the full-fare economy)

Alternatively, if you have so much flexibility about where you are starting from, why not just book an overnight stay in MAD so that you can book your flights individually without risking a missed "connection".

Also note that booking a full year in advance doesn't really work for anything other than flights to/from MAD, until the short-haul schedule is confirmed.