Pretty much it. You could take a tube (Underground) from LHR to Tottenham Hale (change to the Victoria line) and catch the STN Ex from there (not sure what the fare is but it is less than GBP 13 from Liverpool Steet) but the underground fare (GBP 3.70) will probably negate the savings given the hassle factor.
If you are flyingon FR, it used to offer discounted (and restricted) returns on the STN Ex but I am not sure if this is stll being offered.