40% is the maximum discount BBCSO has ever had. The only times when a Spitfire library is discounted more than 40% is with the special bundles during sales, and the Apex sale in the spring with one library offered at 50% off.
BBCSO has never been in any of the special sale bundles since its release, and I think it is doubtful it ever will. It would sort of go against the marketing for BBCSO as a cohesive orchestra.
Having it in a future Apex sale at 50% off is a more likely possibility, but it would be a 6 month wait even if it is the very next one.
So I would say go for it at the current 40% educational offer.
During the Black Friday sale, Spitfire offers a free and exclusive library for any purchases over $349, so if there are other libraries that you want to buy soon, it might be worth a wait to see if the free offer is of any interest. But I think individual libraries are only 30% off during the Black Friday sale.
The Christmas sale is 40% off though, so if you don't get it right now, you won't have to wait too long before you can get it for the same price. And if you don't already have a free copy of Discover through the survey and two-week delay, you can do so and save a few more bucks during the Christmas sale.