score:2
Appendix V to the immigration rules says:
Visitors Appendix 3. Permitted activities for all visitors
Volunteering
4 A visitor may undertake incidental volunteering (i.e. the main purpose of the visit is not to volunteer), provided it lasts no more than 30 days in total and is for a charity that is registered with either the Charity Commission for England and Wales; the Charity Commission for Northern Ireland; or the Office of the Scottish Charity Regulator.
This applies to anybody on a Standard Visitor Visa and anybody from a visa-exempt country, such as the USA, who is visiting the UK without a visa. It sounds like volunteering is the main purpose of your visit, so I think you need a Tier 5 Temporary WorkerΒ β Charity Worker visa.