If you're on that side of the pond, I'd imagine the Ashdown would be a no brainer. That said, I've only personally heard the Rumble and the Mark Bass over here. Both have sounded phenomenal.
I do not care for the sound of MarkBass, and I think they are way overpriced.
Haven't heard or played through the Ashdown, so no comment. Love Fender Rumbles, and the Fender Rumble 500 is killer in every respect.
I use a 121H for practice and for some rehearsals and have been very pleased with it. I enjoy the hint of vintage tube vibe it has and the horn is nicely integrated for a very smooth and natural transition - much different to the 121P. I wouldn't use it alone with a loud rock band, although with an extension cab you may be fine. It is very schleppable if you add castors, although the same might be said of the rumble which gets a lot of love here.
I am very sensitive to tone and a tube addict and the 121H is the first combo I have owned over too many years to tick most of my combo boxes.
As others on the left side of the pond have said, I have no experience with the Ashdown. When I purchased my Rumble 500, I compared it to the Markbass and an equivalent Ampeg (sorry, don't remember the modle name) and purchased the Rumble strictly on tone.
That the Rumble was half the price if the Markbass was insignificant.
I have an Ashdown CTM-30 and am very pleased with it. Good quality parts, and well made. Little better quality build than the Markbass or Fender IME. Still, the latter are very good performers as well, and build quality is more than adequate. I really think that it comes down to your preferred tone.
Interesting question.
They will all sound different to one another and have different strengths.
You would need to try them in person.
Personally I would get the mark bass especially if you can't try any of the amps. Its a safer bet imo (for highly debatable reasons). But its the most expensive.
The new rootmasters seem to be getting good reviews. Also in the UK especially Ashdown has very good customer service. I've only seen the new rootmasters in shops and never tried them. I've heard/read that the newer drivers they use are apprently better than the older blue line ones. And they are supposedly are a upgrade on the mag line they replaced. Although I like the old mag line.
The Rumble v3 series are all good amps. Light weight and seemingly well built. Although I personal don't like the tone myself, but you might though.