Under UK copyright law, any significant work automatically attracts copyright protection. This is designed to protect the rights of authors. Authors (or their publishers) can take legal action against those who infringe their copyright. Your thesis may contain material that you don't hold the copyright for, for example: lengthy quotations, images, photographs, graphs, tables, maps, etc.
If you have included such material from either a published or unpublished work you will need to have obtained the rights holders’ permission to include their content in the online version of your thesis. Third party copyright material can be included for examination purposes (for which the audience is limited to your supervisors, markers and external examiners), but not for open access without permission from the rights holders.