Some fellow online was insisting that London is only the capital of England; I wanted to find some great evidence that could demonstrate otherwise but I couldn't really find any.
The nearest I think you'll find is the 1707 Act of Union which combined the parliaments of England and Scotland into a single parliament of Great Britain. This parliament of course now sits in London, but this was not specified in the Act. Nor, as far as I'm aware, anywhere else. So the government could, in theory, choose to move its operations somewhere else and effectively change the capital, without any need for legislation. As an aside, this has been proposed recently. The Palace of Westminster, where Parliament sits, is in need of major renovation, and one of the options under consideration is that the Commons and Lords move elsewhere for a couple of years - this would most obviously be somewhere in the vicinity, but one MP advocated moving everything to his Dudley in the West Midlands - his constituency. See this newspaper article
I'd also question your friend's statement that London is the capital of England. England isn't a sovereign state, and doesn't have a government of its own (or monarchy of its own) so I don't think it's meaningful to say it has a capital at all.
Follow up question - was London the capital of England before the Romans invaded?