Hi Sally,
You have every right to be confused. Copyright in films made before 1 June 1957 is complicated. Basically a film before that date was not recognised as having copyright except in the individual components of the film, that is, a series of still images each of which was treated as a photograph, the script if it was a work of fiction, and a sound track (if it had one) was a treated as a sound recording. Prior to 1 July 1912 the owner of the copyright in the still images which made up the film was the director or camera operator (or his employer if he worked for a company) and the writer was the owner of the copyright in the script, where there was one. Obviously in your case a Pathé film was not scripted as such (ie it wasn't a dramatic work) and it would not have had a sound track so we are only concerned with the still images.
Prior to the 1911 Copyright Act (which came into force on 1 July 1912) the duration of copyright in a photograph was the lifetime of the author plus 7 years after his death (section 1 of the Fine Art Copyright Act 1862). This changed after the 1911 Act (see
section 21) to a straight fifty years from the date of the making of the photograph (strictly speaking, the negative from which the photograph was printed). Under either of these provisions copyright in the still you want to use ended many years ago, probably in 1961.
Which brings us to the BFI and its terms and conditions. First of all, we can be fairly sure that any copyright in the film had lapsed by the time the BFI acquired the film. At some stage they will have made a digital copy of the film. This is a purely mechanical process which doesn't attract copyright, although it is possible that there was some human intervention to enhance the finished digital copy. The still you want to use will almost certainly have been taken from the digital copy made by the BFI. This digitization process is unlikely to have created a new copyright, although such matters have not been tested in court so it's not possible to say this with absolute certainty (see the section headed "Are digitised copies of older images protected by copyright?" in this
Copyright Notice produced by the Government's Intellectual Property Office). Undoubtedly the BFI's strongest suit is the fact that it can control access to the both the original film and its digital copy, hence any fees they charge probably reflect this aspect rather than being licensing arising from copyright. Institutions such as libraries and archives frequently abuse the copyright system, but to be fair to the BFI the word copyright does not appear in their section on Content Sales and Licensing options, although it is perhaps implied.
However as they have made this one image - the one you want - available of their website, they have relinquished their control over access to that single image and thus you can copy it without infringing any copyright, since none exists. I can't say if quality of the screenshot* will be good enough to blow up for a poster, but if it isn't you may be forced to order a digital still from the collection using their
inquiry form, and of course pay a fee for the pleasure!
I hope this helps.
*
Although I found the listing for the film you mention, I couldn't find the actual still you mentioned, to check its resolution.