TransSec (the body responsible for railway security) and British Transport Police impose strict conditions on the provision of left luggage facilities at railway stations. These currently include a requirement that all items may be subjected to X-ray examination. There are only a few stations in London which have such equipment. (Charing Cross, Euston, King's Cross, Liverpool Street, Paddington, Victoria & Waterloo). All of these facilities are managed by the Excess Baggage Company and they all close at 2300. (Victoria Coach station also has X-ray screened facilities but, once again, they close at 2300).
The Metropolitan Police use powers under anti-terrorism legislation to enforce similar rules upon any other locations wishing to offer left-luggage facilities. (i.e. lockers are banned because property can't be X-ray screened first). So, unfortunately, it's extremely unlikely that you'll find anywhere in central London offering left luggage storage at the hours you want.
Chris