State of Play 23 Feb 2015
The Home Office has prepared a set of substantially revised visitor rules which will be laid before Parliament this Thursday (26 Feb 2015). The new rules, along with the new guidance, will answer questions like yours with much more clarity and resolution than the current rules/guidance. The rules will be unveiled on the Hansard site this Friday (27 Feb 2015) and activate in April. The Home Office site will be updated during March.
Part of the new rules will be introducing volunteer work as a permitted activity for visitors (tourists). Specifically...
incidental volunteering for up to 30 days at a registered UK charity;
"Incidental" is used here to mean the volunteering is not the main purpose of the visit. If the main purpose of your visit is to do volunteer work, you will not qualify.
"registered UK charity" means an organisation listed on the UK Charity Register. Oxfam, for example, is listed on the register, so a visitor can do up to 30 days volunteer work at Oxfam so long as the volunteering is incidental to the person's visit.
Another example is the World Wide Opportunities on Organic Farms (WWOOF). Their entry in the Charity Register is here.
The Charity Register has a search function that enables users to determine if a given organisation has a listing or not. If you cannot find an organisation's registration entry, it means they do not qualify.
NOTE: Although volunteer work will be permitted, a visitor will not be able to serve as a director or trustee because these roles occur in other legislation.
ALSO NOTE: The new rules will activate in April 2015, you cannot benefit from them until that time.
ALSO NOTE: As always, it takes a long time for everyone to learn about new rules (including Immigration Officers), so it's recommended to print out the relevant text (when it is published on the Home Office site) and carry it with you. The rules will be posted at Immigration Rules in April 2015. Guidance will be posted on the IDI site.
ALSO NOTE: A listing on the Charity Register is not necessarily a permanent thing. If the organisation loses it's registration, you will have to stop volunteering there or face possible sanctions.
There are additional changes affecting visitors, but these are mostly about business visitors. Finally, the UK legal community was advised about these changes two weeks ago by a letter sent from the Home Office's policy unit; they should be ready by now to give advice if you need it.
Adding 26 Feb 2015
As laid before Parliament earlier today, the new rules are here. The relevant text for this law is...
A visitor may undertake incidental volunteering, 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