As others have indicated, the only left luggage facilities you'll find at railway stations are staffed offices (which are run by independent companies). Security considerations mean that many stations aren't even allowed to have any form of litter bins, yet alone luggage lockers.
Where staffed offices exist they must X-ray all bags, which means that expensive equipment is needed. Consequently costs are high. Victoria Station's left-luggage office was charging £8.50 per item (for up to 24 hours) when I was there a few weeks ago.
To check on whether there are left luggage facilities at any particular station, enter the station name here:
http://www.nationalra...tations_destinations/
Then click on 'General services information'
Chris