Hi I have a publisher requesting I send a TestFlight to their account but I thought that would mean me registering the app on my iTunes account and ready for publish. Is this correct?
I am not 100% sure, but I think that it is correct, since you can't upload test build to ITC without registering the app on ITC account. Anyway if they decide to sign your app you can easily transfer your app to their account - few clicks and it is done.
Make a test-app on your account (name: irrelevant) - fill in the BASICS only, - upload the beta game and fill the spot on iTC then try to activate test-flight -- it will probably error out or something, but it will send a request to Apple people that you'd like to activate a testFlight for external testers -- at least this was my experience. Then in the next 20-30 hours TestFlight & External Testers will be activated -- so you can add e-mails of people who'd receive the game for testing.
Interesting as testflight with external users is set to be available after review only. As far as I know, officially you can bypass the review only by using internal users. If it works for testflight external users without review, I'd consider this a "bug" or better: luck I guess the fast beta app review might not be as thorough but officially it has to pass the review, even for the external user test. Therefore fastest approach is internal users (require an ITC account) but with a very limited quantity of testers.
Just use an outside service like https://www.diawi.com or setup your own process for doing this. Google around a bit. No need to wait for Apple if you don't want to. But it's definitely a more technically advanced option.
You can do an ad-hoc distribution, just ask the tester / reviewer for their UDID so you can add them to your certs
