Some CYMA AK's are around the £120 mark - they're better than the G&G Combat Machine M4's for range and possibly for accuracy also. But hey, if you want a little machine pistol, maybe you should hold off buying a gun for a while, rent a few times, get to meet people and have a quick fondle/shoot of their guns (we love our guns and will nearly always rise to any occasion to show them off), see how different styles suit you.
If you still want a machine pistol then, for anything decent and working, you might be best buying 2nd hand, if "cheap" is a huge issue. Leaving aside the MP7, there's MP5K, Mac11, or Mac10 that have been around long enough that you see them for sale. G3 SAS maybe (just not the Bell one), M4 pistol (no buffer tube, like the G3 SAS, it doesn't exist in the real steel world), or an AKS 74U.