The way that has worked best for me is having the BA low price finder page open.
Then when the cities are announced at midday, I set the trip length and navigate to the city.
Click into the city and select the month/s that the offer is for. The calendar of dates is loaded with prices, but the prices are incorrect, so just ignore those.
Click your date, and through you go to the BA booking area and you'll then see the correct £100 fares listed. If the date you selected has none, then you will see on the tabs the prices for days either side, and if any of them have £100, your in luck.