I would not be so fast to conclude that this is an error. The same behaviour can be seen on other airlines' fares. For example, taking LHR-SYD as a random QF route, all fares in W, R and T classes are classified as "E" except for the highest W class fare. Or taking LHR-HKG on CX, all fares in W, R and E classes are "E" except for the highest W and R class fares (which are both in price).

I have a suspicion that it's done this way for some techie reason such as to avoid these fares being unavailable to the markets in which premium economy is a poorly-known and poorly-recognised concept (yes, USA, I'm looking at you in particular). Otherwise, if you ask for "economy" fares in a market like that, you might never be offered a BA T class fare even if it is cheaper than the cheapest available (true) economy fare. That would be a lose-lose for both BA and the customer, and classifying these fares as "economy" may be a way to avoid some problem along these lines.