Does the UK monarchy have power?

British people often say that the monarchy of the United Kingdom of Great Britain and Northern Ireland does not have any power. That it is a symbolic institution which brings in a lot of tourist revenue, and serves as a figurehead only.

Other people say that the monarchy is necessary because it holds the ultimate power, to override the elected government if it felt it necessary to do so.

Are there any experts on constitutional monarchy here who can shed light on the truth here?

Thanks!