You will need to have both your rail tickets and your 2-4-1 voucher at the attraction so to keep your tickets at the station, you need to go to a manned barrier and ask to keep them, the automatic barrier will keep them otherwise.
I think that would qualify , the direct Thameslink service goes through Blackfriars and St Pancras .
So, will West Hampstead do, or do I need a ticket for a Z1 terminal? Any help?
"do I need a ticket for a Z1 terminal?"
Blackfriars and St Pancras are Z1 terminals, you will be passing through both.
In other words you will be travelling from Gatwick in to a terminal and out again.Edited: 30 March 2018, 15:54
*turn light switch on*
You can come out of the room.