When I had a Z1-6 travelcard season, and wanted to try out a 395 to Ashford and back, I got a BZ6 to Ashford ticket and paid the HS1 supplement.
I can't remember how I got through the gateline, but I got on the train and everything was fine until the TM said it was invalid as you can't use Travelcards (even with a supplement) and charged me an excess. The excess wasn't that much, mind.
To me it's mad that you can have a station (or two) outside the zonal system and a line where Travelcards aren't accepted at all.
I don't understand that. What is the purpose of the HS1 supplement other than to upgrade you from "not valid on HS1" ticket status to "valid on HS1" ticket status?
My partner has an outboundary to Z1-6 Travelcard season and has paid the HS1 supplement on days she's been running late to travel on HS1 without problems?