GHK. They do a lovely M4 and several AKs and the G5 is possibly one of the best on gas efficiency. Is you fancy something a little different and are into customisation then i highly recommend the KJW KC-02 (which I currently own and possibly could be tempted to part with but I love it so it wouldn't be cheap with all the bits done to it).
Of the GHK guns, the G5 and the AK74 (from Milspec solutions) will fit your budget. Remember to factor in cost of extra mags which will be another £100-£150~ on top.