I don't know whether
sillies (and
silly as a noun that can be made plural) is specifically British. But it is quite rare, and I don't think it should remain as a common word.
Searches on
sillies turn up a lot of references to
The Three Sillies, an English folk tale that appeared in an 1890 book called
English Fairy Tales, collected by Joseph Jacobs. A few years ago an updated version of the story, under the same title, was published by US children's writer Steven Kellogg.
A recent appearance of the word in literature was in
The Stray Sod Country (2010) by Patrick McCabe: "O you men! she chuckled... You really can be such sillies at times, do you know that?" The book is set in Ireland in the 1950s.