You have got it all wrong. The movie was shot in England (Manchester I think from memory because of its resemblance to New York during the 1940s), but Florence was American.
Here are a few suggestions if you would like to see Meryl play an English character/person: The French Lieutenant's Woman, Plenty and The Iron Lady.
Per Wikipedia: "Nascina Florence Foster was born July 19, 1868, in Wilkes-Barre, Pennsylvania, the daughter of Charles Dorrance Foster, an attorney and scion of a wealthy land-owning Pennsylvania family."
And she didn't have a British accent, but rather what's known as the "mid-Atlantic accent" - a blending of British and American English accents which was cultivated by the upper classes and people in the theater and film industry in the early 20th century. Katharine Hepburn and Bette Davis are good examples.
She could also have picked up some phrasing and intonations from her long-time British husband.
And she didn't have a British accent, but rather what's known as the "mid-Atlantic accent" - a blending of British and American English accents which was cultivated by the upper classes and people in the theater and film industry in the early 20th century. Katharine Hepburn and Bette Davis are good examples.
I think the OP is being coy. Meryl Streep has played so many Brits in her career, that she along with Julia Child, and Vincent Price, were Americans whom were always thought of as British.