It was a glitch. Basically it would have been a PE seat with Y service as I don't think PE is sold on this route.

Usually PE seats are blocked for airport control and they are given to elites (CX or OW) but sometimes the odd PE seat is open for selection to general passengers (even those without status).

This only happens when an aircraft with PE is used on a route that doesn't have PE seats sold, so PE is basically part of the Y cabin.

It has nothing to do with you having redeemed a ticket using BA miles.