Southend-on-Sea is the closest major beach town from London, especially when you're travelling from East London (Underground to West Ham or Barking, then 1 hour by train in the direction of Shoeburyness). Southend offers a nice alternative to the glitzier, but also gaudier and much more crowded Brighton - and if you just want to spend a few hours on a relatively quiet strip of beach (stony though) near the North Sea, I can safely recommend it. In the last four years it has become my habit to bookend trips to London by going to Southend and walking the 1.34 mile Southend Pier, the longest such pier in the world.
The pier itself is also much more quiet than the one in Brighton . Except for a really nice café with outside seating, there are just a few small food stalls here, without the cheap amusements, casinos and carnival. It is truly a pier that you walk along to soak in the maritime atmosphere, and in good weather it offers splendid views. And if you're too lazy to walk, you can always take the short tram ride at half-hour intervals (during the summer, price for an adult is UKP 5.20 for a "return ticket", or the walk & ride option for 4.70). Entry to the pier without the tram ride will cost you UKP 2. To give you an estimate, it usually takes me 25-27 minutes to reach the end of the pier without stopping and with no children to slow me down (If you're with kids, expect 35 minutes at least).
The pier and its view have always reminded me of "Shell Beach" in the movie Dark City. I especially like the café, which offers small cakes, scones and sandwiches and a modest range of (non-alcoholic) drinks. From its elevated terrace you have a nice view onto the pier. In rainy or extremely weather there are enough seats inside.
There is a lifeboat station which includes a teeny, tiny museum and a small gift shop. Sitting options to relax, unwind and enjoy the view are plentiful. The pier is in good shape, frequent repairs to the wooden surface are evident from the varying color of the planks.
Note that at ebbtide you have to walk about a mile on the pier to even reach the waterline. read more