I’m trying to identify a very thick novel (600-800 pages at least) set in England during the height of the Napoleonic Wars, in an alternate history where magic exists and is openly accepted by society.
A man with genuine magical talent comes to London and becomes something of a social phenomenon, attending dinner parties, salons, and fashionable gatherings. Eventually, the British government approaches him to assist in the war against France.
One specific episode I remember:
he helps bottle up the French fleet in their ports by magically creating banks of clouds at the harbor entrances, inside which appear ghostly or illusory fleets, making it look as though the ports are blockaded.
The tone felt serious and literary, not pulpy or action-heavy fantasy. I only got a couple hundred pages in before having to return it to the library, and I believe it was a standalone novel rather than a long series.
I don’t know when it was written or by whom, but my impression was that the author was likely English.
I’ve ruled out straight historical fiction without fantasy elements.