You have to file US Income tax forms as you a US citizen and you will have UK tax withhled from your paycheque. I'm not sure you would have to file Italian tax forms as you don't live there (check with them), the US is the only country that I know that requires you to file a tax form even though you don't live there.
Since £25000 is less than $80000, you most likely will not have to pay any US imcome tax, but you still have to file.
There are many threads on this topic here.
bvamin