I have no realistic alternative to using Thameslink. And I'm not prepared to:
I don't know your personal circumstances, but you almost certainly do have alternatives. They will take longer, be less comfortable, or cost more, or any combination. But they exist. And you will choose to use Thameslink as it is the best combination of price, time and comfort for your circumstances.
My alternatives:
5 hour walk (free except for shoe wear)
80 minutes bike ride (free except for tyre wear, and considerably less safe)
60-90 minute drive (much more expensive, and less reliable)
50 mins moped trip (would have to buy one...)
15 min drive or half hour bike to a zone 6 tube station for a 45 min tube trip (cheaper, but not much if driving to the tube)
30 mins on Thameslink with a walk either end.
Except walking, all are realistic. But I choose Thameslink. If it gets bad enough that the cost / time / comfort equation makes one of the others more attractive, I'll switch. But it's unlikely.