At least it could be a HST rather than a Voyager with Virgin!
Clearly taking a detour is going to add even more time to the journey than either going direct or via London (you are paying a premium for the privilege of travel via London BTW). There are some vaguely sensible alternatives but none will leave you much time to look around.
Via Manchester is permitted following Plymouth - Birmingham - Manchester - Leeds - York (either route) - Newcastle. Via Carlisle is not permitted.
There's a few variations on the cross-country route permissible (but maybe not practical), such as going via Westbury to Bristol, Worcester to Birmingham or via Leicester for Birmingham-Derby. You can't however use the Marches line to Manchester.
Plymouth-London can be via the Berks & Hants, via Bristol and GWML, or via SWML from Exeter.
London-Newcastle can be via the ECML (optionally via Cambridge and/or Leeds and/or Sleaford/Lincoln) or via the MML (optionally via a load of places in the E Mids and S Yorks).
Remember to take into account the
ticket restrictions when planning your route. This seems to be a very permissive ticket, with no restrictions between London and Newcastle (unless I am missing something), and no restrictions at all if not travelling via London (although good luck trying to use it on XC before 0930).