Odd train / season ticket costs

DroidSkin

This is really strange.

I'm looking at getting a train season ticket from Ashford to London.

The quickest route to my work is on the high speed train to st pancras. The season ticket cost is £6228.

As it happens, my current client is near another mainline station, so I can get the slow train direct with a season ticket cost of 'only' £5192. It's only a little slower door to door.

Bit of a no brainer it seems.

But.....

There is an option to go to London via Haywards Heath at a cost of £4668. So I looked up the route for that.

And the route it says is to get the high speed to London, tube over to Victoria and then train back out!

Which got me thinking, what's to stop me just breaking my journey at st pancras?

In fact a season ticket to Haywards Heath from Ashford would only be £2604 so I could buy that?

I'm obviously missing something somewhere?