Where a choice of routeing is available for SVRs for a flow, and both are priced by Virgin, then the Virgin website, and one or two other websites, seem to be happy to sell two SVHs with different routeings for each leg when purchased at the same time, thus obviating the need to find a staff member willing and able to sell excess fares.
There have been cases on the forum where passengers have run into trouble when attempting to excess a fare after boarding, especially when travelling via London on a 'not London' SVR.
That said, excessing the return leg for travel via London would actually be a slightly better option, since the return portion would be valid for a month rather than a day as would be the case with a SVH.
I generally try not to go into too much detail about 'loopholes' on the open forum, but one SVR/H, from a station north of Stoke (and valid via Stoke) for travel to Portsmouth, has restriction code 2T and the other, for travel via London, has restriction code 2C.
If needing to return via London during the evening peak, then it would be possible to buy a slightly more expensive 'not London' 2T SVR from an even more northerly station to Portsmouth, excess the return leg (only) to a 3A SVR, and stop short at Stoke.