I don't think 'panhandler' is known in UK. It makes me think of me preparing the dinner in the kitchen! And 'beggar' is rather perjorative. I can't think of a word which describes the activity but avoids being a little insulting. I like Kampervan's view that you give in good faith and most of the recipients are genuine. If not, it won't kill you to have given in vain. I used to work with prisoners, and one or two were very scathing about beggars. They said it's much more dignified to steal and burgle than to beg. Couldn't quite follow their logic on that one!