The UK Mapping Festival takes place from 2 to 7 September in London: see their sponsored post on Londonist. The festival is a mix of exhibitions, talks, workshops and other events. Exhibition passes are free, but certain events are not. The conference part of the festival takes place over three days and costs up to £95 per day to attend, but as the program is a mix of geospatial, historical, cartographic and general-interest material, you might not need to go every day.