Apologies if this isn't the time or place, but it's too late to withdraw from anything, IMO. That damage was done by Blair and the UK supporting the overthrow of Iraq.
We, and most of Europe as allies of the USA, have left a vacuum there which was swiftly filled by the Daesh (ISIS) along with Libya, Syria, wherever else in that region, and other weak and volatile places such as central Africa, and allied attacks on them in Iraq and Syria since, has spurned these revenge atrocities, whether "justified" or not.
Women and children have been lost to "both" sides, so maybe in their eyes, it's just "tit-for-tat" and will be forevermore.
None of this is any comfort at all for the victims of these terrorists and their families, but understanding the causes and avoiding more blunders in the Middle East (or anywhere else for that matter) will be the only way to contain it at the current level and maybe over the next generations, bring it to a close and remove the potential for it to continue.
We've unleashed a monster in that region, and it won't go away without it biting us more.