I need models for London IT event

I'm not going to pretend I understand what you want these people to do, but ... if you want young then see if there are Jobshops attached to any of the local colleges and universities who will advertise for free (some charge!) They may say you've left it too late, so you may need to put postcards up in the nearest union buildings. Or shops around the venue.

Oh, just remembered, our local Uni Jobshop lets you fill in the forms online, so that might help you suss out where to advertise.

Of course students travel miles to get to Uni in London so you've got a huge choice!