A big name in this regard is SwapaholicsUK, they organize events at least once a month in Brixton or Shoreditch and every second Saturday of the month at Grafton Pub in Kentish Town the system is easy: you bring one piece and you take any other ,whatever it might be: you can drop a sweater and take a coat or bring a trouser and grab a belt, everything has the same value here. With a mission of reusing and recycling everything you can, Swapaholics also organizes events where people can swap books, accessories, household items, amongst other things. Entrance fee is 5€ or 4€ if you buy it online and if you subscribe to their newsletter you will be aware of all their events.
Swishing is another similar company even though things work differently here, you must at least bring one item half an hour before the event starts and then you can take as many things as you want. You can even organize your own event as well, as it is an idea under the collaborative economy and aims to expand around the world.