At least 800 British Muslims have left the country to wage jihad with Islamic State. Another 600 were caught trying to join the group. If that isn’t warning enough about the failure of Britain’s three million Muslims to assimilate, a new survey of British-Muslim attitudes should be.

The survey of 1,081 adult Muslims, conducted by the research firm ICM using face-to-face interviews, will be featured in a documentary on Britain’s Channel 4 on Wednesday. The Times of London has published some of the findings, and they make alarming reading.

Thirty-one percent of respondents believe that a British Muslim man should be permitted to marry more than one woman. Fifty-two percent believe homosexuality should be illegal. Twenty-three percent support replacing British law in some areas of Britain with Shariah law, the system of Muslim jurisprudence. Under its strictest versions, a woman’s testimony and parental inheritance rights are legally worth half of a man’s, and adultery is punishable by stoning.

More than a third of Muslims think Jews have too much power in Britain, compared with 9% of the general population. Meanwhile, 7% of respondents support the establishment of an Islamic state, and 4% sympathize with the idea of committing terrorism “as a form of political protest.” Those last two figures seem small, until you consider that they could total some 100,000 to 200,000 British Muslims.

Some Muslim organizations dispute the results. The pollsters concentrated on communities where 20% of the population is Muslim. These are some of Britain’s poorest neighborhoods, critics argue, and the pollsters were more likely to interview the country’s least-integrated Muslims, skewing the results.