Can we do volunteer teaching (no pay) at charity school with DTV

I'm a freelance designer doing remote work pro bono for a charity based in Indonesia, which I visit a few times a year. I am living off savings so have money in the bank. The charity is willing to write me a letter describing our relationship. But since they are not paying me, will this be an obstacle to me getting a visa? I have a portfolio of work related to my role with them. I will be applying at the London Embassy.

I'm a two dimensional visual artist, and I've planned a project in Thailand, painting and photography, of the people and places. After selling a particular piece I wish to share a percentage of the proceeds with the subject, and with a charity. I'm an American citizen and I believe a 60-day tourist visa is appropriate, but after reading stories about "working" in Thailand, I'm concerned. Should I apply for something else? A business license, perhaps?