5.56 is prohibited bore, unless you have a PB licence (quota is full) no point even trying to buy one as the penalty is pretty harsh. But just for discussion sake, if you do hold a PB licence a M4a1 will cost you about 300,000 (Armalite) with a few shiny accessories. M16 A2 will cost about 150,000/200,000 depending on quality etc etc... Also be careful when dealing with vendors if you are new to buying weapons, you can get stung pretty badly. So when going shopping, take a local with you who knows the inns and outs.
The sensible choice if you are buying for self defense is the "shorty" Benelli_M3 which can set you back 100k or a Zigana 9mm which will cost 25,000 and gives you most bang for your buck. If you are buying for practical Rifle shooting, you can pick up a .222 Armalite copy for 120,000 from Majid & Sons or any half decent dealer in Kai/Lhr/Rwp or Pesh.