I don't personally find £12 a year that bad, compared to most things, for a lack of ads in an app I use a lot. I pay about the same for Appolo to browse Reddit.
I can understand feeling butthurt if they told you you had lifetime status and then reneged, though. I bought lifetime membership to LingoDeer for learning Japanese and have kept that over two subscription model changes. That said, we're talking about a fair bit more money than TT charged for their lifetime membership there, but you could also argue that that isn't the point.