イギリスでボランティアするために必要なビザはTier 5 (Temporary Worker - Charity Worker) visaとよばれます。このT5のビザを取得するにあたり、UKBAが定めていることを原文のまま案内いたしますので参照ください。

1. Overview
You can apply for a Tier 5 (Temporary Worker - Charity Worker) visa if:

you want to do unpaid voluntary work for a charity
you’re from outside the European Economic Area (EEA) and Switzerland
you meet the other eligibility requirements

Sponsorship
You need to be sponsored - ie have a certificate of sponsorship from a licensed employer - before you can apply to come to the UK to work.　チャリティ団体からの受入書が必要

The work you do in the UK must relate to the work of your sponsor organisation.

How long it will take
You can apply for a visa up to 3 months before your date of travel to the UK.　3ヶ月前からビザ申請可能です。

You should get a decision on your visa within 3 weeks.　3週間程度で結果がわかります。

Check the guide processing times to see how long getting a visa might take in your country.

Fees
How much you pay for a Tier 5 (Temporary Worker - Charity Worker) visa depends on your situation, where you are and how you apply.

￡208　2014年4月現在の料金です。これは日本で申請した場合の料金です。

How long you can stay
You can stay for up to 12 months or the time given on your certificate of sponsorship plus 28 days, whichever is shorter.　ビザの期間は最長12ヶ月です。

You can enter the UK up to 14 days before the start date of your job.

What you can and can’t do

You can:

work for your sponsor in the job described in your certificate of sponsorship
do a 2nd job in the same sector at the same level as your main job for up to 20 hours per week
do a job on the Tier 2 shortage occupation list for up to 20 hours per week
bring family members with you

You can’t:

receive any payment for work
take a permanent job
get public funds