Just to be clear - I am no stranger to day trips - as an adult I can recall multiple ones to Paris (by train), Frankfurt (by air), Geneva (by air), Turin (by air), Barcelona (by air) and even New York (strictly speaking less than 24 hours but I did fly Concorde so I thought I'd mention it
) but these were always planned some way in advance whether for business or pleasure.
Never have I woken up in Leytonstone on a Saturday morning (or even Friday evening) and thought,
"I wan't to go to Paris for the day today/tomorrow and the only thing that is stopping me is the lack of a suitable walk-up fare" and I suspect I am not alone.