According to film historian Robert Osborne, Sellers portrayal of Rev. Smallwood was based on (and is a "dead-on" impression of) one of his teachers - Brother Cornelius - in Catholic school.
At one point when Rev. Smallwood (Sellers) is preaching from the pulpit, a young boy is seen in the foreground reading a copy of Nabokov's "Lolita". Sellers played Quilty in Stanley Kubrick's film version of that book.
Steve Marriot, who played Harry Smith's son Jack, was later to become the lead singer and guitarist with influential English rock group, the Small Faces.
The voice over at the beginning is Peter Sellers.He also provided one of the voices heard on a television sequence.
There are several well known 60's British TV personalities playing themselves including Ludovic Kennedy and Malcolm Muggeridge.