I took the Underground Accelerator for a spin last night and had a blast.
It is a great pedal and a worthy successor to the Particle Accelerator - can't comment on the London Underground since I never played one.
One thing, though: People described the pedal as a more capable overdrive than the PA, which is supposed to have more clean headroom.
My PA, which is an old unit from before the 12AX7 tubes, reaches unity with the gain around 9 o'clock and the master around 10:30.
My UA reaches unity with both dials at noon.
With the UA's gain set to max, I get a pleasant sound that can be described as warm and fat, much like a tube amp that is just on the brink of distortion but not yet there. There is not much grind to speak of. The middle of the fretboard is quite clean, the upper registers get singing overtones and the lower notes (below A) get that farty texture when the bass knob is dimed (which it is). Is it supposed to be like that?
I play a passive Le Fay 6 string and in my default playing setup, the preamp is the first pedal in line.