It is frustrating to read your post. Everything you describe in terms of symptoms and your MS screams "OPTIC NEURITIS HERE!". Aside from being in London for a week and loving every bit of it, I have no experience with the UK or the UK's medical system. That being said, it seems that you are not receiving the prompt treatment that you need. Yes, the ON is a relapse. There is no reason to assume otherwise. You need quick treatment with IV and oral steroids to stop the inflammation as quickly as possible to shorten the course of the inflammation and to hopefully lessen the permanent damage to the nerve that it can cause.
I am glad you are receiving treatment now. Good luck, and if this reoccurs in the future, pester your medical care providers and get treatment