In this case they went to William Hill, an 80 year-old publicly listed and regulated company that made >£200M in profit off £1.5bn revenue last year.
The counterparty risk was negligible, especially compared with the rather high risk the pollsters all made systematic errors in their surveys (it's not particularly difficult to imagine a scenario in which people with a stated intention to vote for independence turned out to be much more likely to actually bother to vote; actually the main reason why I didn't try the lower risk, lower return version of this bet and place all my liquid funds in a FTSE100 index fund or ETF the day before)
The counterparty risk was negligible, especially compared with the rather high risk the pollsters all made systematic errors in their surveys (it's not particularly difficult to imagine a scenario in which people with a stated intention to vote for independence turned out to be much more likely to actually bother to vote; actually the main reason why I didn't try the lower risk, lower return version of this bet and place all my liquid funds in a FTSE100 index fund or ETF the day before)