A while ago I bookmarked this page:
https://www.railplan2020.com/frequently-asked-questions which says
"Customers who wish to travel to any London Thameslink station including London St Pancras, Farringdon, City Thameslink, London Blackfriars, London Bridge and Elephant & Castle may do so by purchasing a London Thameslink ticket where available. Customers with London Thameslink tickets may also break their journey at Kings Cross to provide greater flexibility, however they are not valid on the Moorgate branch."
Having just checked it today it seems the javascript is broken and clicking on the questions doesn't expand the answers, so to get that text I pasted above, you can either disable javascript, do a view-source, or look on the Internet Archive:
http://web.archive.org/web/20180518114329/https://www.railplan2020.com/frequently-asked-questions