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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/options/sectors/psychology-careersPsychology 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).