There's some mistake in the use of wording on BA website.
I redeemed on JAL flight GMP to KIX last month using BA Avios, BA website says it's premium economy when I search economy, but obviously Premium Economy is not offer on the route, it's just mistake in wording. Most importantly, it charged the rate correctly at the rate of economy, 4500 Avios.
If it's a short route, PE not offered, just ignore the wording.