There are many opportunities to get involved on a voluntary basis with science communication activities.  Two organisations that are at the forefront and provide interested parties with excellent voluntary opportunities are:

Researchers in Residence
Researchers in Residence (RinR) brings together researchers, young people and teachers via exciting and innovative placements in secondary schools and colleges across the UK. RinR enriches the classroom experience, engages young people with real-life research and gives researchers the opportunity to develop new skills. RinR benefits everyone involved. Open to all secondary schools and colleges in the UK. Open to all PhD and post doctoral researchers funded directly or indirectly by one of the seven UK Research Councils or the Wellcome Trust.

STEMNET Science and Engineering Ambassadors
The STEM Ambassadors Programme enables teachers to make links from the curriculum to how STEM is practiced in the world of work, illuminating applications across a vast range of careers.
STEM Ambassadors are people from STEM backgrounds who volunteer as inspiring role models for young people. They can contribute both to regular lessons or participate in extra-curricular activities such as STEM Clubs, Careers Days and visits. STEM Ambassadors open the door to a whole new world for young people, helping them to see STEM subjects and careers with a fresh perspective and engage their interest and imagination in new ways.
Each Ambassador is registered, trained and CRB checked, ambassadors receive ongoing support trough offered training courses and advice. A STEM ambassador does not need to be an early career scientist, people who are enthusiastic about science are heartily encouraged to get involved!