well as resident phone booth expert, I've never seen a poster in one, and as said above, not sure where it could possibly go.
I have photographed booths in:
Thailand,
England,
Scotland,
Ireland,
Canada,
USA,
Caribbean,
Dubai,
and of course Australia.
Don't know where you went in the UK, but I've seen loads of telephone boxes with film posters on them. They are a peel off the protective paper, and stick to the glass kind of thing, so they can be viewed from the outside.
We also have Lamp-post posters, that fit in a light box half way up a lamp-post. The poster size seems an odd size on these, almost 1sht size..
I'm wishing I'd taken some pics of these if only for Ari's records/collection. Next ones I see...