My previous email bt yahoo was advert free in the folders - inbox, sent etc but as from yesterday without the choice irrelevant adverts which cannot be deleted. Not a good idea. Please return me to the previous format. If it ain't broke don't fix it.
BT don't have any say in the matter - it's a Yahoo decision. Their profits are down recently, so they need to show even more adverts to try to make more money.
You're assuming that BT are actually paying Yahoo. But in any case, Yahoo makes the decisions and BT pretends that they are introducing it as an "upgrade". That's always been the relationship between BT and Yahoo.
After all, what can BT do? Threaten to take their email business elsewhere?