This question is somewhat related to this one, but not quite the same.
I'm an Italian citizen currently living in the UK, and I'll be traveling to Spain in about a week (from Heathrow). My passport (which I always fly with, no matter where I go) is in the process of getting a visa, so I won't have access to it for this trip. My only option is to use my Italian ID card (which looks like this, and I have already used to enter the UK from Italy); will I have any problem going to Spain and / or back to the UK?