Yes, you can claim benefits while volunteering as long as you don’t get paid or receive any rewards or benefits in kind for your work. You can still get reimbursed for expenses as long as they’re actual out-of-pocket expenses and not pre-estimates. Reasonable pre-estimates are only allowed when you will pay back the surplus money after the expense has been incurred.
If you’re on benefits you will need to tell your job centre about your volunteering work and about the expenses you will get reimbursed. It’s your responsibility to do this and not the responsibility of the organisation providing the volunteering work.