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There are many York-based and nation-wide voluntary opportunities listed in the Clinical Psychology section of the webpage below, but do also search for your own.
https://www.york.ac.uk/students/work-volunteering-careers/ideasoptions/sectors/psychology-careers/
You are encouraged to gain experience early on, but please make sure to balance carefully any volunteering with academic and social lives (you are unlikely to gain entry into a DClin course without good grades).