The only booking office I know has closed over the last decade is Hilsea near Portsmouth, as far as I know no others have gone*. FCC wanted to close Drayton park and SE wanted to close Martin Mill about 3-5 years ago, however these were rejected by the DfT. That said however, London Midland have been given permission to close booking offices at Cheddington, Lye, Witton and Wythall, but I'm not sure when they will close.
Many booking offices at the smaller halts across the land went in the 1960s, with smaller numbers in the '70s. The last large cull was between 1988-1993, when about 50-70 closed nationwide. If anyone has any specific info they want I'll try my best to source it, as I'd quite like to know too.
Quite a few have however been restaffed in recent years, for example Worle and Nailsea & Backwell in Somerset, Burscough Bridge in Lancashire, and many on an independent basis, like Ludlow, Chester-le-Street and Newtown (Powys).
* - That said, NXEA introduced booking offices at Attleborough, North Walsham and Wymondham in Norfolk on a trial basis in 2008, however these didn't last and they closed last year I'm afraid. Nearby (-ish) Saxmundham in Suffolk used to be run as per Ludlow etc independently, however this closed without replacement a while back.
(Yes, booking offices are my special interest!)