No rail stations, anywhere in the country, have had left luggage lockers for many years. Stations may only store left luggage if they have staffed facilities, with the ability to X-ray or hand search all bags. Very few stations offer such facilities; however, all London mainline termini stations can store left luggage.
Where left luggage facilities exist, they are not provided by Network Rail or the train operating companies; they're franchised out (which means that they're run strictly for profit, rather than customer convenience). So the charges are quite high. (The last time I heard a figure quoted, it was �6.50 per item. So, if you've got several bags, try to put the smaller ones inside the larger ones).
The facility at Kings Cross is run by the Excess Baggage Company. It's located by Platform 8 and it's open from 0700 to 2300 daily. To check that they're still open (and to find out their current prices), phone them on 020 7837 4334.
Chris