Ordinarily that may be true, but I am flying out on a very busy weekend - gay pride in Madrid, which is the best in Europe. Thousands of others will be flying out for that, thereby avoiding the special BA Square set up for the equivalent event in London.
The flights coming back to any London airport on Sunday and Monday are very busy (almost all already £200-£350 for that leg and some showing zero availability). Not all the flights going out look so bad so maybe things are a bit more staggered, but that LCY-MAD flight (Thursday evening, 13 days ahead) is currently showing a price of £345. I'm guessing that means it looks fairly full, although I don't know.