The genuinely fast services will be on HS2, the legacy WCML will be, like the German legacy routes, or the Swiss routes bypassed by the NBS lines[1], about capacity and connectivity, and Pendolinos may indeed not be the best stock for that. Quite possibly something like a 110mph OHLE Class 444 or even 350 would be better.
[1] The NBS (Neubaustrecke = new build lines) aren't true high speed, but conventional stock, mostly LHCS, running at about 225km/h. But the effect is basically the same, and the concept - providing capacity and higher speed "in parallel" to an existing line - is basically the same.