I first saw this type of thing in Paris. Couples visit the Pont des Arts over the Seine, write of their undying love on a padlock, chain it to the bridge and then throw the key into the Seine never to be retrieved and the padlock locked forever. All very romantic.
Now some bright spark has used this idea (just in time for Valentine’s Day) to recreate the concept in Covent Garden. I assume you throw the key down a drain rather than into the Seine or keep hold of it should things not work out as planned.