A person came up to the cosplay desk this May with a strung compound bow and real arrows, asking us if these were okay. Granny's response was to take the bow off them and ask them, "Is it okay for me to shoot you with it, using those arrows?"
You can guess what they said in response to that XD
That was just one person who came up to us though, think how many other people are walking around with that sort of thing and not actually realising that it's a hazard, or who deliberately flaunt the rules because 'everyone else does it'. If the bow is brought along with no string and no real arrows then that's fine, but accidents with strung bows and real arrows happen even on archery courses, it's not the kind of thing you want to think about happening at Expo. An arrow shot at close range from a compound has the potential to hurt multiple people depending on where it hits :\
Also, there were actually police vans outside the ExCeL this year and the police were confiscating things as well as security. The Woolwich machete incident made them feel the need to monitor people closely this year. Needless to say, they had a field day.
The moral of the story being, read the prop and weapons rules and stick to them, and email if you're unsure and want to clarify anything. Get in touch in advance of the event rather than a few days before, also - there were a number of props/costume enquiries the day before or during the event that it was not possible to answer because internet access at the ExCeL is limited due to the overloaded wifi and phone networks.
I'm derailing this thread so I will stop there.
I just Emailed them now, about bringing in my Jason Voorhees prop machete, it is in a holster so it will not be carried around, its made of plastic, but looks identical to the movie, we will see how they respond about props like this