Accoriding to the routing point calculator you can use either Liverpool Group or Wigan Group at the gathurst end and either Guildford or Effingham junction at the Horsley end.
Liverpool Group to Guilford allows
LONDON, CH+EJ+PD, CH+RJ EV+PD, ME+EB+PD, ME+RB, NO+LC+RD, NO+RM, NR+LC+PD, NR+RM, RV
Wigan group to Guilford allows
LONDON, EW+PD, RW
Liverpool Group to Effingham junction allows
LONDON
CH+EJ+DK, CH+EJ+EF, CH+EJ+HR, EV+DK, EV+EF, EV+HR, ME+EB+DK, ME+EB+EF, ME+EB+HR, ME+RB+DK, NO+LC+DK, NO+LC+EF, NO+LC+HR, NR+LC+DK, NR+LC+EF, NO+LC+EF, NO+LC+DK, NR+LC+DK, NR+LC+EF, NR+LC+HR, RV+DK
Wigan Group to Effingham junction allows
LONDON, EW+DK, EW+EF, EW+HR, RW+DK.
I'm going to focus on routes from Wigan, both because I belive it's the more practical start point (using Liverpool as your origin routing point would mean travelling via southport) and because the lists of routes for Liverpool are crazy long. Lets start with the routes via London.
Wigan Group to London group allows.
BS+GC,CL+EJ, EW, PR, RW+RG, XW+PR
So in terms of routes from Wigan to London you have quite a few options.
BS+GC is WCML routes (including via Manchester) to Birmingham and then Chiltern to London.
CL+EJ is via warrington, Chester, Crewe and then various WCML routes to London.
EW is WCML routes to London, including the possibility of going via Manchester and/or Birmingham.
PR allows travel via Manchester, Sheffield and the Midland main line.
RW+RG allows travel via Birmingham, Oxford, Reading and the GWML
XW+PR allows you to travel via Warrington, Manchester, Sheffield and the MML
In terms of (non-london) routes from Wigan to Guilford you have
EW+PD is WCML routes to willisden junction, then willisden junction to Clapham junction and onwards to Guilford.
RW allows travel via Birmingham, Oxford, Reading and then direct to Guilford.
In terms of (non-london) routes from Wigan to Effingham junction you have.
EW+DK is WCML routes to willisden junction, then willisden junction to Clapham junction and onwards to Effingham Junction via wimbledon and epsom
EW+EF is WCML routes to willisden junction, then willisden junction to Clapham junction and onwards to Effingham Junction via surbiton
EW+HR is WCML routes to willisden junction, then willisden junction to Clapham junction and onwards to Effingham Junction via micham junction and epsom
RW+DK allows travel via Birmingham, Oxford, Reading and then via Guilford to Effingham junction.