A post from Graculus on a Facebook forum, might be of interest to Simon......
Recently British Telecom contacted Groundspeak (who own geocaching.com) after an engineer had found a couple of caches inside phone boxes. BT said whilst they didn't want to discourage geocaching their engineers would remove any containers they found and asked that no more be placed. After discussion with Groundspeak the UK reviewers have decided that no more caches will be published which are in or attached to phone boxes. Any active caches currently in place won't be archived but we would ask owners to move them to a new location as soon as possible to avoid them being removed. Caches that subsequently go missing or are in need of maintenance must be archived rather than replaced or repaired.
Please note that this only applies to phone boxes (of any type) operated by British Telecom and not to any operated by other telecommunications companies such Kingston Upon Hull Telecom, Manx Telecom, Jersey Telecom or Cable & Wireless (Guernsey). Nor does it apply to any of the old 'red' phone boxes that are out of use and may be owned and maintained as attractions by local councils or parish councils. For these normal permission would apply.