Hi! I graduated from Psychology in uni this year! If you want the experience to help you get into a psychology degree, what matters the most is that you really need to express your interest and motivation for choosing psychology in your personal statement. I heard others say the same and many people on my course had 0 experience when they started. (Might be because of covid at the time for me but I don't think that should affect your chances)
However, if you really want to gain experience, it would be good to do some volunteering, like from charities that support mental health, special needs or vulnerable people. Since you want to become a therapist you will need to have the ability to interact with diverse people, something to show that you care for the community, and communication skills too.
I was also really lucky because I knew someone who worked in a special needs school and was able to arrange for me to work shadow them (I ended up assisting a bit bc they were short staffed one time). I am also aware NHS does some work experience weeks for y12-13, heard they are competitive but I think it will be great experience since its exposure to the NHS.
They don't have to be very obviously related to psychology or mental health, but really give it a think about how your skills and different experiences are going to help you as a psychologist. Hope that helps!