I doubt Intamin ever would make anything with switchbacks. It's not in their catalogue, and they have a relatively solid business in making "traditional" coasters, as in "after the train somehow has gotten enough speed, it all about track". If people order anything from Intamin, they'll most likely get the traditional "get speed" section (launch or lift), followed by track, then brakes. No mid-ride gimmicks.
And B&M won't even make launches. I can't imagine them putting together a switchback. Not now.
My bet is Mack or Vekoma, or perhaps S&S.
Worst case scenario:
The Alton executives have designed the ride themselves (on NL), ordered track, theme and construction for £4 mill, and the last £8 mill is budgeted to be fixing design flaws and maintainance the coming three years. Basically, what SFMM would have done if they could see the future, but were forced to install X.