When I was last at University about 15 years ago I used to volunteer at the café at St Martins in the Fields in Trafalgar Square on Sunday mornings serving food to the homeless. In my experience most were suffering from 'issues' relating to mental health problems and or drink/drug dependence.
It only takes a bit of 'bad luck', often involving family breakdown and taking away their support network, to put people on the streets, and many of our clients were people 'just like us' including a disproportionate number of ex-Forces who had become institutionalised and seemed unable to cope with organising their day to day living, when no longer told what to do and when to do it 24/7.
I have since made a point of never giving money to beggars. People say they will only spend it on drink or drugs. I know that I probably would if I was in their position! But I will often buy them a coffee or a sandwich.
Since Uni however I have set up a standing order to make a monthly donation to St Martins. Here's the link if anyone wishes to do the same:-
https://www.connection-at-stmartins.org.uk/donate-on-a-regular-basis/