Twickenham Film Studios, St. Margaret's, Twickenham
A great deal of post-sync work was required of the Beatles during post-production of Help!, overdubbing speech (clear as well as plenty of incoherent muttering and "ho-hos"). For the sequence (shot May 9th) in which John and Ringo were filmed on their way to a post-box they even overdubbed an extract from "I Sat Belonely", one of the pieces in John's first book In His Own Write.
Source: The Complete Beatles Chronicle - Mark Lewisohn