Photographing the homeless

I posted a this photo to a Facebook street photography group and got this question.  So i'll post my response here too.

"I took a similar photo a few months ago and uploaded it to a local website. I had hundreds of messages from people telling me I shouldn't upload photos without their permission (I don't know if you have too) but as above I can see no wrong in it Can anyone tell me the legal or moral position because I left the other website as a direct result of the abuse and threats I received."

My response: In the UK you can take a photo of whoever you like if you're taking it from public land. You can then use the photo for anything aside from using their image to sell a product. The question I would say is can you justify it to yourself on moral grounds. I chatted to this guy afterwards, listened to his story, gave him money and when I saw him later I bought him a hat to wear. This wasn't any sort of agreed exchange but I like to feel I treated him as a fellow human being and didn't take his photo or not take it based on his homelessness.

Any comments on this?