Has anyone made a definitive guide of what scenarios it is and when it isn't part of Southern?
Going well off topic. But I have a personal experience on this after some research and concluded some rather interesting discussions. On the occasion in question I used a "SOUTHERN ONLY" ticket on a Gatwick Express service and was told by
RPI▸ it wasn't valid. An interesting argument started over the
NRCoC▸ which states a "Train Company" and not "Brand Name"
Whilst this may be a moot and pedantic point, it actually isn't. Looking at Gatwick Express' Website it's very clear to me from some wording hidden in the footer notes as follows:
^ Gatwick Express is operated by Southern
Southern Railway Ltd is registered in England No. 06574965
With the emphasis in bold. It's operated by Southern Railway... So "SOUTHERN ONLY" tickets should be valid as Gatwick Express is merely a copyrighted trademark/brand name and not a separate Train Operating Company with a Company Registration Number...
Write a strongly worded letter to Southern if you get charged an Excess by Gatwick Express... They refund you in cash every time it happens!