This appears to be the relevant website:Spending the day in London before heading back tomorrow evening. LNER site says you can pre-book luggage storage at Kings Cross to collect later but there’s no mention of how you book. Any ideas?
Indeed, I’ve just come across this company too. They seem to have self service lockers local. (https://stasher.com/luggage-storage/united-kingdom/london/kings-cross)You might want to look at Stasher as a cheaper alternative. They store in places like Premier Inns.