Volunteering outside the UK
You can volunteer for 2 weeks or for a year or more and there are opportunities all over the world. But finding the right placement requires a lot of research.
VSO they place experienced volunteers of any age and also have a programme for 18-35 year olds www.vsointernational.org
Charity Job advertise UK and international roles www.charityjob.co.uk/volunteer-jobs
The opportunity to volunteer abroad is offered by lots of organisations many of whom are profit-making and charge volunteers.
International Citizen Service (ICS) is a UK Government funded programme providing volunteer placements to 18-25 year olds and Team Leader placements to 23-35 year olds. Find out more here.
The Telegraph produced a useful article reviewing gap year providers including charities in August 2016 http://www.telegraph.co.uk/travel/gap-year-travel/gap-year-100-companydirectoryvolunteering/
University College London also have some useful guidance
The UK Government has issued safety guidance for those seeking more adventurous volunteering