I always found it strange at Paddington that the platform wasn't annouced until a couple of minutes before the train arrived "for security reasons".
I've never heard a delay in platform advertisement described as being for security reasons. Platforms are planned alongside the timetable (months in advance) but at a terminus like Paddington there are a couple of obvious operational reasons to advertise "late": the train is being prepared for its next journey (cleaning, seat reservations) so is not ready for boarding and, when things are not going to plan, it avoids last-minute changes causing confusion (maybe ticket barrier issues at Paddington specifically).
At non-terminus stations, the first reason isn't applicable, and scope for platform changes is often limited by the infrastructure so platforms are displayed as far ahead as the departure board goes.
See also eg http://www.realtimetrains.co.uk/search/basic/PAD.