One carbine that's based on the M4 is the Rock River Entry Tactical model:
http://www.rockriverarms.com/m4_entry_tactical.htm
Because of the shortened stock and 16" barrel it's a very manageable size. You may also want to invest in a good collapsable stock and a pre-ban upper (with flash supressor) since I'm assuming that the brady bill has no relevancy over there. I purchased one of these at a gun show for under 800 and it's by far my favorite rifle. If you want to fire with iron sights you can, otherwise you can remove the iron sights and use it as a flat-top to attach your favorite reflex sight. Good clean fun.
Good luck over there, always be vigilant and hope you never need to use your weapon.
Jason