generally ignore it tbh...
i live in London and AFAIK there is enough sheltered accommodation etc.. to cater for our homeless population
the only people begging or sleeping rough, I'd guess, have drug problems or are con artists and either way I think it is a bad idea to give them money
I actually get quite annoyed by the 'professional beggars' on the tube - they change carriages at each stop and always seem to get a couple of quid from each one - basically they're earning a pretty decent wage out of being a parasite.