When Dr Who is translated into Japanese, do they translate police box as こうばん? I've seen こうばん in Japan, and they aren't anything like old British police boxes, so if that's how it's translated, is that confusing to a Japanese audience? Or do they use some other term?
First to deal with the Japanese language related question as to whether we could use 交番 to refer old British telephone boxes: As a result of the discussion on these pages I would say yes because although there seem to have many types of British police boxes, they were on the whole very similar in function to the 交番 to be found in Japan.
A Japanese 交番 is more like a mini-police station manned 24/7, or if you like somewhere between a glorified sentry-box (where a soldier would stand on duty) and a guard-hut, rather like you find at gates, where there might be one guard watching out on duty and perhaps another in the back room which probably contains a small office and place to rest. My dictionary give the expression 交替で番に当たること which I translate as "to guard in shifts".
We can see from this site: henderson-tele.com/policeboxes/policeboxes/bytown.html that Dr Who's Tardis was only one a huge variety of police boxes in Britain, and although the main function may been for police officers and the general public to contact their local station in the early days of telephones and portable 2-way radio was portable, they also functioned as a "local police office". (The Tardis type was smaller than many but also fell into this category.) googling "交番 明治 photos" reveals that early Japanese 交番 have also varied in design and size but given that they usually also functioned as a "local office" 交番 is probably correct expression.
Second, to deal with Dr Who and translation of Tardis: When the show was put on Japanese TV several years ago it was referred to as a ターディス. The wiki-link (http://ja.wikipedia.org/wiki/ターディス) and related wiki-entries explain Dr Who, what the Tardis is and where the name comes from. I am not an expert on the show or the Tardis but, as a time machine (タイムマシン）, its ability to travel through Time and Relative Dimensions in Space (TARDIS) seems to lead to it being referred to as an asteroid (小惑星）of some sort. The real fans can probably explain better than I but:
If you google "Dr Who Japanese" several pages come up including some English features in Japanese papers. Police boxes are vary rare in the UK now so they don't mean much to many fans there either but one article suggests:
"The fact that on almost every corner in Tokyo there is a Police Box (otherwise known as a Koban) and some of these places actually say 'POLICE BOX' on the front, is unlikely to have been lost on NHK. Expect some intergalactic police advertisements soon."
So to summarise: The Tardis is known as ターディス in Japan. People are familiar with time travel etc but its guise as a police box probably means about the same (ie is as confusing) to Japanese fans as it must be to its British fans under 40 although the existence of 交番 in Japan might be of some help. The old British police boxes were, on the whole, very similar in function to the 交番 to be found in Japan and therefore 交番 is probably the appropriate expression.