Originally Posted by lexxity
You will need your passport to present to the immigration guys on your return.
Moreover, you will need your passport to enter Schengenland on arrival from the UK. So the airline will not permit you to travel without a passport.
Only travel entirely within Schengenland is OK without a passport. You have to get there first.
So I agree that it's off to the Passport Office. The good news is that they are very efficient these days.