Twickenham Film Studios, St. Margaret's,Twickenham
Its marked contrast to the films A Hard Day's Night and Help!, which had been finished and released a few short months after going into production, How I Won The War was a much slower affair. Although shot through the autumn of 1966 - John completing his role on November 6th - its world premiere did not occur until October 18, 1967 (at the London Pavilion, with all four Beatles in attendance).
During the long interlude, the film's post-production processes were undertaken, with post-sync dubbing of voices and music being done in the Recording Theatre here at Twickenham from this date through to March 3rd. John attended at least once - though a precise record of date(s) no longer exists - overdubbing his voice onto the soundtrack.
In an opportunistic, if not downright cheeky, attempt to promote the film, the United Artists record company released as a British single on October 13, 1967 - a few days before the premier, a "song" called "How I Won The War," attributed to Musketeer Gripweed and the Third Troop, Gripweed being the name of John's character. The clear inference was that this was a John Lennon single, issued without the other Beatles. In fact, it was almost without John Lennon too, for the 'song' combined the film's theme music written by Ken Thorne (who had done likewise for Help!) with a melange of effects and dialogue extracted from the soundtrack, and John was evident for less than two seconds.
Sourc: The Complete Beatles Chronical - Mark Lewisohn