Until the early 1970s, Liverpool - Shrewsbury - Hereford - Bristol was the normal route for N West to S West trains. Personally, I think a better option might be Liverpool to Plymouth trains, rather than Liverpool to Southampton via Bristol.
Plus some Liverpool - Birmingham - Oxford - Southampton - Bournemouth / Pool trains, taking over a few of the existing XC paths south of Birmingham, and LM paths north of Birmingham. . .