I noticed that you baked them in a 9" square pan. All the ones I ate in England looked like they were individually baked in a deep muffin mold, so I assumed the batter needed close contact with hot metal to help the rising/gluten setting effect.
Do you think that might have impacted why yours didn't rise as much as you thought?
Do you think that might have impacted why yours didn't rise as much as you thought?