Looks like it is more than a storage shed. 1st image is from 2012 2nd from 2020.
https://www.alamy.com/stock-photo-london-england-uk-theatre-ticket-kiosk-in-the-west-end-57700790.html
https://www.alamy.com/theatres-and-sightseeing-tours-closed-in-london-during-the-coronavirus-pandemic-lockdown-may-2020-uk-image356959639.html