I thought there was a page on the BT Help and Support website that said what the upload speed for Unlimited Faster Broadband was, but I can't find it now, all I can find is
What is Unlimited Faster Broadband?. Anyway, it's not the same things as this new 18/2 FTTC, but I suppose that could change in future, although BT Consumer saying "we'll always make sure you get the highest possible speeds your line can support" implies they might use a 40 Mb option in case the estimate was too low.