If you're after miles and not SCs, per se, the following change would net you 4,844 miles more base-miles:

Instead of:
...JFK-lax-dfw-sea-dfw-phx-LHR... (13,168 miles for this portion of the itinerary)

Fly this instead:
...JFK-LAX-SJO-LAX-SAL-LAX-LHR...(18,012 miles instead, plus a few visits to sunny Central America)

Only caveat is that I believe LAX-SJO is seasonal.