There is an article
here about working with Crisis as a volunteer dog handler. This enables homeless people to get much needed respite and care for their canine pals. This was last year in London but there might be something similar in your local largish town.
Alternatively you could ask at at your local rescue centre if they want some extra support
A friend of mine volunteers to help feed the homeless in Manchester every year. She says it's the most rewarding thing she's ever done and that everyone should do it at least once (like Jury service), she says it should be compulsory!