You see people begging a lot on trains and tubes if you travel into and around London. From those leaving packets of tissues and then coming back and asking you to pay for them, or those saying they only need one more pound for a bed in a shelter. Some of the latter have been very aggressive and threatening.
If you really want to do something, give something, donate to a homeless charity. I don't even bother with Big Issue these days. I work in central London and the women selling it near my work are a gang, who are run by a man. I have seen them very early in the morning being directed to different streets. They also approach people outside cafes and pubs like wandering vendors rather than static at official pitches.
There also seems to be a new scam where they are kneeling as if they have no legs and they are rocking, except they do have legs. (Fair play to them even if they are scammers, must take it out of you and must be better ways of earning a living).
Or they have a St Bernard (or another large mountain dog) with them. When they see you looking they invite you to stroke them and then ask you for money. Near Euston recently I saw a homeless guy sitting begging with a blanket over his legs and on a mat thing. He looked dirty and unkempt. He then got up and his bottom half looked completely unrelated to his top half. He was wearing good shoes and a pristine pair of Armani chinos. Homeless top half only....you get cynical after a while.
I do see genuinely homeless people on my walk to work. I come into Victoria station and around the area there are lots of people sleeping rough. Tents are appearing, they are sleeping on old mattresses. Which is why I say give to homeless charities who can target where the money goes. Giving to scammers, however well meaning, does not help the homeless.