I'm super confused because my understanding was any BA premium economy ticket was eligible for upgrade.

Furthermore why did the BA agent not tell me it was not an upgradable ticket but instead go ahead and start looking for availability and calculating the cost?

Purely theoretical now for my understanding, if you buy a premium economy ticket lets say LHR-JFK and its selling class is E or T you cannot upgrade it to club with avios, only a premium economy ticket with W selling class can be upgraded to club?