That would prove you hold British citizenship, but doesn't prove you aren't a citizen of another country which seems to be what they're asking. Not sure how you'd do that.
Having said that it only says confirmation, not supporting documents. I got an information request about my relationship with the mother of my children, I just sent a letter confirming the facts, didn't include any supporting documents and it I think it was fine.
I'd write a brief letter confirming your citizenship (say it's your only citizenship if that's the case), can't hurt to include your passport but I assume they already have a copy of that.