Four years as a HCA will stand you in good stead, especially if this is MH HCA work. In terms of voluntary work, it really depends what you want to do after your degree. I'm assuming clinical? If so, there's loads of stuff you can do; helplines, day centres, befriending, volunteering with people who have TBI/ABI or ASD/LDs, ABA tutoring for kids with ASD, holiday clubs, respite work, addiction drop-in centres, homeless shelter. Search on do-it.org and there will be loads and loads of stuff.