Pretty much none of the organisations can get enough volunteer visas. Most volunteers are taking a risk in Thailand to be honest. I’d get your retirement and if you volunteer, fly well under the radar. The type of volunteering they are really against is people who ‘volunteer’ in a profit making business like hostels in return for food and board.
You go to the UK Gov website to print out your forms and check the supporting docs you need. Then send an appointment request. UK Visa Application Centre
I used to admin a big group but most people don't notice it or look for it I don't think. Anyway, I couldn't be bothered to do this level of herding so good on you for not taking any shit.