As always littlewoman is right on the money, I'll just re state the need for a torch, get one from Ebay the led 'creed' ones are brilliant, only a coupla quid each and I have loads. The torch will be a great help when you've landed to the site at 12am and you're trying to pitch your tent in the pouring rain and pitch black (get one that fits in your mouth) Do not listen to the wannabe security guys who say "I have a 4 cell" that's not needed and if they ever hit anyone with it they'll get a short trip for a long stay.
Don't pitch your tent at the bottom of a hill, always get the highest ground you can, water flows downhill, pick up left behind tent pegs when you leave, they're handy.
I use a two room tent, that's maybe not the best idea because someone always wants to bunk with me and they bugger off when it's time to take it down and go home.
As for blow up mattresses, I've never used one, I'm hard - or else too lazy to carry it lol.
Your badge holder, get a viper, it's leather and fits 3 ways, belt, lanyard and arm. I always wear mine on my belt if I'm on doors or wearing tshirts but i have to wear it on my arm if wearing a coat, I hate wearing it on my arm but that's just me, I always had a habit of taking my coat off and forgetting about the badge and had to go searching through the coats to retrieve it.
Bog roll, Bog roll, Bog roll, can't say it enough, it's needed. Baby wipes are handy too.
Cheap mobile phone, one of the old ones, they stay in charge for days but bring your charger too obviously, you will find there's only about four or eight sockets for 300 security guys, I'll never forget the look on a guys face who was massively built and quite a bit taller than me when I walked in and unplugged his phone, it was disbelief lol, I then took a double adapter out of my pocket and plugged both in, so bring a double adapter with you.
Painkillers, I got one hell of a toothache last time in London in the middle of the night, even if you don't need them someone will, (don't worry about the first aid kit)
Earpieces, as TMAC said get the covert ones, one for Motorola and one for Kenwood, the company may supply one but, 1 them cheap plastic ones are damn uncomfortable, 2 god knows whose ear it's been in before and 3 when you get suntanned you will look silly when you take it off having one white ear.
Everyone has given you good advice here but I'm gonna state the obvious... your SIA badge!!!, we had a numpty who forgot it once and couldn't work till it was posted to him, also had a plum who forgot his passport once too.
Now, if you're camping put any valuables, phone wallet etc at the bottom of your sleeping bag at your feet when you sleep, you will find out about tent thefts soon enough. When you are on patrol or deployed I always keep my passport in my combats, don't actually know if anyone else does that but when I'm not in the tent all my valuables are on my person.
One last thing, on patrol or response, help the good looking girls put up their tents, it pays off
Good luck and welcome to the fold mate,