This is free, and a classic, esp for BrE:The King's English
H.W. Fowler
http://www.bartleby.com/116/index.html
It may be old in some areas.I'm also using:Curme, George O. (1977). "A Grammar of the English Language". Verbatim. ISBN 0-930454-01-4 (reprint of 1931 edition...