I'm sure some probably do. Even in real life, some people are homeless by choice & still have the means to get by with the usual things.

Good point, I've never thought about that, but they obviously do.As LoveBug has said, virtually all phones have cameras these days, so it would not be a huge investment. It would also allow Sherlock to contact them when needed.

The quality of the pictures taken with cheap phones is still usually quite low, though. I have a model which was top of the range in 2010, and the camera is ok outside or in a brightly lit room, but there is no flash, so you can't use it in the dark or even somewhat badly lit places.

I'm always a bit sceptical when they use pictures taken via camera phone on TV shows, with people using them as evidence or for black mail. It's just a bit unrealistic, at least not for "regular" people who don't spend 300+ EUR on their phone.
Irene Adler's case was slightly different, thankfully the writers put in the extra bit of information that her phone had been adapted by Moriarty's "experts". Also, she is rich and well connected enough to have a state of the art camera put into her phone especially for her purposes.

It's true... I'll go with the theory that Sherlock gave them good camera phones.
It's also true that the pictures you see on screen have a way too good quality. The quality of cell phone pictures is usually very crappy. But you can observe that in almost every movie or TV show - pictures someone "randomly took" always look way too good to be authentic. It's just one of these little mistakes which are perfectly fine.

I'm less surprised that the homeless have camera phones than then having phone plans. Does Sherlock pay for all their plans?

Do any companies in England offer a "any of homeless" deal?

I was wondering this too! They must be pay as you go. And I personally don't see Sherlock paying for them? I guess they are a somewhat good investment but he's not super rich either.

True, maybe Mycroft even set it up.