Well, having done London to Newcastle and back today, both on Lumo services, here's my observations.
On the outward journey, I had two airline seats to myself, and the journey was very comfortable. I was fortunate in that I had the aisle seat reserved and no-one showed up for the window seat. Leg room was fine and I'm 6ft 3".
On the way back things were different. I was again stuck in a aisle seat, and one without a window view. I booked almost immediately it went live, so it's nice of them to flog the crap seats off first! Anyhow, I had someone next to me this time which drastically altered my experience. Having to sit upright for the whole 3 hours caused me quite a bit of discomfort. I could not lower the table tray without stretching my legs out- it's ever so close to being ok if they could have raised it just one more inch. The seats backs are very high and you end up staring at the blank dark blue seat back when you have no other view! Maybe they should have went to whole airplane hog and put TVs in the seats backs and charged for watching?! 15 quid for each leg seemed just about right. Any more, I'd rather pay the extra and go with LNER.