Now that 377 layer onto WCML, why not run them up to St Albans Abbey and then plan to let them come down from East Croydon thru Clapham Junction ? In the case of the 710, they layer in for their full run from Watford Junction into Euston Sta.
The 710 layers in and does a full run because the line it runs on (the Lioness Line) runs from Euston to Watford. The 377 is a different slice of cake. They ain't going to extend the route down south as the WCML starts at Euston. Wouldn't be much of a WCML if they built the line to Clapham Junction as well.