... London ESB ... serious under-attenuators... I find that the beers made with these yeasts are still quite tasty, even when under-attenuated.
I would just like to point out that Danstar describes London ESB as:
"Attenuation range 65-75%"
So if your beer attenuates to 65% using Danstar's London ESB, it would not be considered "underattenuation" but I would consider this a low attenuating yeast strain. Underattenuation would be when a yeast strain falls below the brewing properties described by the yeast manufacturer; and overattenuation would be when a strain exceeds that range. I guess that's my take on the terms "under/over attenuation". BTW, I've brewed three beers with London ESB and finished with expected attenuation levels of 65%AA, 67%AA, and 71%AA (ALL were exceptionally tasty and full-bodied beers!).
For me, it's the attenuation levels of this yeast that makes it so unique. I also feel as though it's glycerol production is on the high end, which would be another defining characteristic, but I have no proof of this (just the mouthfeel from those three beers; one of which should have been pretty watery due to low OG, low mash, no caras).