The UK government website for Standard Visitor Visas explicitly says
- do paid or unpaid work
- [some other things]
so that definitely isn't the right visa.
The more detailed guidance says (on page 22):
Appendix 3 allows visitors to undertake volunteering (not voluntary work) provided volunteering is not the main purpose of the visit, it is for a registered charity and will be for no longer than 30 days in total. Where an individual is looking to come to the UK as a visitor specifically to volunteer they must be refused. Volunteering and voluntary workers are provided for in the Tier 5 Charity worker route of the points-based system.
The volunteering is the purpose of your visit (though it is less than 30 days), so you need a Tier 5 visa.