£20.65 Kentish Town - Canterbury West
(Railcard assumed due to previous price quoted)
Cheapest I could find
Mike, unfortunately no railcard for me so Kentish Town comes in at £31.30 so slightly more than St Pancras (which is logical).
I have had a look at most stations on the Great Northern Line, if they are in the London Transport zones (upto Hadley Wood) they are all more than the St Pancras price. Any station including Welwyn Garden City south to Hadley wood costs £28.50. Going onto the Thameslink route all stations are more expensive I looked. Tried Watford Junction and Slough they are all more.
I know the amount of time I have spent looking has probably cost more the £2.20, but it got me interested. Hertford also has a cheaper fare than St Pancras, I guess it is the same zone as Welwyn. If South Eastern are reading I am sure a HS1 only fare will be on its way shortly