Sometimes you just have to get to the sea in a hurry. The water, open horizon, wind, waves, and salty air are things I really miss about living in London. Luckily, the sea is only a quick one hour train journey away to Brighton (also quite inexpensive to get to with a Network Rail Card).
Get yourselves to London Bridge station, grab takeaway breakfast from adjacent Borough Market (Saturdays) and enjoy being speeded to the sea. From the steps of Brighton station, it is only a five to ten minute stroll to the beach promenade. From there you can wander as far as the eye can see. No matter rain or shine, it is beautiful. I almost prefer it in winter, when the clouds are racing and the sea breeze is strong. Be warned though, it is busy with lots of sea-loving types, at any time of year.