Originally Posted by
bhomburg
Do you need/want 'real' Premium Economy - small, separate cabin featuring wider seats with more pitch and recline, bigger entertainment system screens, better food and wider beverage selection, amenity kit, and airport benefits like expanded baggage allowance and priority check-in that's found on AA, Alitalia, Air France, BA, Lufthansa - or is one of the extra legroom products the US airlines offer (Main Cabin Extra at AA, Comfort + at Delta, Economy plus at United good enough? ....
....The easiest airline to upgrade from ecnoomy to Premium Economy for a discounted price is BA, either in advance online through 'manage my booking' or at the airport for cash. The only way to use points for this is using BAs own points system, though - if you don't have Avios, you need to pay cash.
For true Premium Economy and ability to upgrade, I think BA is likely your best bet for what you are trying to accomplish. Only catch is you may have to switch airports in London, from Heathrow to Gatwick.
If you simply want a bit of extra legroom, you can get 1 stop options on AA via PHL, ORD and CLT. Row 25 on their A330's has quite a bit of legroom but are a bit narrower from the traytable being in the armrest.