The answer to this is very clear in the National Rail Conditions of Carriage and I'm sorry to say I disagree with
yorkie.
NRCoC 10 states:
The validity of a ticket may:
a) be restricted to; or
b) prohibit
travel in the trains of a particular Train Company or Train Companies.
"Train Company" is expressly defined in Appendix A as:
a company operating passenger railway services which is required to apply these Conditions to its tickets under a condition of the Passenger Licence granted to it by the Office of Rail Regulation. A list of these companies can be found in Appendix C. “Train Companies” means all or more than one of these Companies[.]
(Emphasis added.) Because this phrase is expressly defined in the contract, it supersedes any traditional dictionary definition of the words.
Appendix C lists the "Train Companies" (as of the most recent 19 July 2015 version) as:
Govia Thameslink Railway Ltd (trading as Great Northern)
Govia Thameslink Railway Ltd (trading as Thameslink)
[...]
Southern Railway Limited (trading as Southern)
Southern Railway Limited (trading as Gatwick Express)
Recognising that this list may be out of date, the appendix also expressly refers to the National Rail website for a current list.
The National Rail website lists the following Train Companies (
http://www.nationalrail.co.uk/tocs_maps/tocs/TrainOperators.aspx):
Gatwick Express
[...]
Great Northern
[...]
Southern
[...]
Thameslink
Thus, for the purposes of Condition 10 of the NRCoC, Gatwick Express, Great Northern, Southern and Thameslink are defined as separate Train Companies, regardless of whether they are in fact merely brands of the same corporate entity.
I agree that this is not ideal for customers, but it is completely unambiguous in the contract.
Clearly these rules should be relaxed when a Southern service is operating in Thameslink livery or vice versa --- and I would hope they would be. But that is a separate issue.
Ben