I'm going to be travelling to Gatwick Airport (0930 Flight) from Finchley
Central on Thursday and returning a week later.

I have a Zone 2-4 Travelcard and a YP, my girlfriend has a Zone 1-4
Travelcard and a YP. We were thinking of getting the first tube and using
the Thameslink from Kentish Town or Kings Cross Thameslink.

Are there special prices if you get a Thameslink only ticket ?

Any other advice on the journey would be aprreciated

Cheers