They wouldn't go unwanted on the Cardiff-Pompey route! Far better than a dreadfully unsuitable Turbo anyway.The 175s are unreliable rubbish (with nice interiors, though needing a bit of a tidy, but they are not reliable). 150s are getting old but are very reliable workhorses.
175s also aren't useful for the high loadings Northern get due to their door layout. They would be suitable for Northern Connect (and indeed have operated those services in the past), but there's already new stock on order for that.
But that's all assuming that this is a full fleet replacement. We've been surprised before.