Have you read the ultimate guide at the top of the forum. It will give you most of the details.

LIS has great special fares, rather than year-round fares, though January deals *should* be available soon. To use the current promo, you need to take the LIS-LON flight before Dec 15 and then "stopover" in LON till Jan, but there aren't too many weekends left. It should then be €2,128++ in club (+€200 for the stopover in LON) or €2,660++ in First (stopover free). You could wait until the new fares, but they could b higher or may not appear.

There are some decent year-rounds about though. I've quickly tried ATH, VIE, FCO and ARN. ATH seems the best at about €2150 all in - it's further to go but easyJet is a cheap option in Dec/Jan. You wouldn't have to do a stop, but if you do it's €200. IST may (travelocity not playing ball this morning) do you a slightly better deal, but more hassle perhaps, and may have an F fare, but it's unlikely to better LIS.