I am not categorically against the idea. But it cannot be a UK/US-led operation. Those countries demonstrated that they cannot be trusted for the better part of a decade when it came to Iraq. The 2003 invasion was illegal, immoral and made the world a more dangerous place. Even if the US and UK could stop ISIS today, it would only touch off more anti-Western, violent sentiment and resentment.
US and UK forces could participate under the behest of some kind of regional alliance or under the auspices of a UN mission to create peacekeeping, enforce international law and take war criminals to justice. It'd still be the height of hypocrisy as long as, say, George W. Bush was not also being taken to justice, but it'd at least be plausible enough.