Why do we poll online?
Three main reasons: We believe that it is the most representative means of conducting research. True, not everybody has access to the internet but not everybody is available to take a phone call at 6.30pm or answer the door when the interviewer knocks. By allowing respondents to answer the questionnaire at a time and place of their choosing you increase the percentage of people available to answer across all demographics. Internet uptake is now sufficient in all sectors of society to be able to sample to cover all sectors of society.
This strikes me as really interesting. If you have to call or visit at 6:30pm, you're more likely to get people who work less, housewives, people who work from home and the unemployed. Workaholics and young successful people with social lives might not be home by then.
So, maybe some of the polls are understating the "parties of the right" (Conservative/UKIP) and overstating Labour.