Or the 5x additional classs 153s from GWR...
Or, 'world-class' short-forms; which appears to be the chosen strategy at the moment...
Although the bulk of the PRM cover is needed on the valleys (don't forget the 158s need PRM work too), that doesn't mean any additional stock HAS to be used on the valleys. If LHCS could be quickly introduced to cover a class 175/1 diagram or two then those 175s could go onto 175/0 diagrams (providing a capacity increase) releasing 175/0s for the HOWL or Conwy valley to release a 150 or two for the valleys. Of course, ideally you would want any LHCS cover to replace a 4-car class 150 diagram, which of course would only exist in the valleys (or possibly at a different TOC, with the released 150s subleased to Wales).