Thats correct.
Im travelling on a Thursday at around 11am then returning in the evening at about 9.30pm
In that case, you can turn up on the day at the station and buy your ticket from one of the two card-only machines outside, or two cash and card machines inside, or from the ticket office (if it's still open by the time you visit).
Often, London Zones 1-6 Travelcards will be on the first screen of the ticket machine, but sometimes these aren't the best value ones.
Instead you can press the "A-Z destinations" button and search for "London Tra" and the option for Zones 1-6 should be on the right hand side of the screen.
You can then press "cheapest ticket" at the bottom and it should select one for £25.50.
a Day Travelcard including travel from Haywards Heath to London and back, and unlimited travel in zones 1 to 6, is 25.80
You can actually get a cheaper
super off-peak travelcard for £25.50 that allows travel on both Thameslink and Southern up to Zone 6 (effectively East Croydon).
This is valid for trains arriving into London from 10:45am onwards, but (in a nutshell) you cannot use it to leave London back to Haywards Heath between 16:14 and 19:16.
I would personally recommend this one if you're not leaving until about 8:30pm (to arrive back to Haywards Heath at around 9:30pm) simply because it gives you more choices for services to and from Haywards Heath into London (the £25.80 one is Thameslink only).
I won't go into the conversation about whether "Thameslink only" and "Not Gatwick Express" is actually valid or enforceable. It's not worth a tourist worrying about.
It is worth mentioning the "Thameslink only" and "Not Gatwick Express" bits on the ticket
only apply outside of Zones 1-6. For example, if you choose the Thameslink only ticket and want to go to London Victoria, you can take Thameslink to East Croydon (which is in Zone 5), then change for a Southern service to London Victoria.
