upcoming.org is a website that you should definitely utilise since its free.

other than that, i suppose you should frequent some london-based forums and let them know what your proposing…

p.s. you can also add the event on this website if you haven’t already as well.

You only need a few groups of people to make it worthwile, although they are not easy to get. There are lots of places you can leave notices such as backpackers but those people are not likely to plan that far in advance or go to expensive places which yours might be. Word of mouth is the best method as every person knows other people and if a person wants to go then they want other people they know to go I would suggest networking and getting other people to work for you as much as possible.