Useful websites
You may find the following websites useful.
Prospects - https://www.prospects.ac.uk/jobs-and-work-experience/gap-year
The graduate careers website offers advice on gap years.
NCVO - https://www.ncvo.org.uk/ncvo-volunteering
National Council for Voluntary Organisations website.
Volunteer Services Overseas (VSO) - https://www.vsointernational.org/
The UK website of this global organisation.
There is also a wealth of specialist gap year travel agents websites for you to look at.