As a US citizen, you always need to file a US return if you meet the filing threshold, and your worldwide income is reportable. If you had income from the UK in 2012, that should have been reported.
No, you don't send in a P60; it's sort of an 'honesty system' - your UK employer won't be sending info to the IRS.
You can use either the earned income exclusion after meeting the bona fide residence test,or take a federal tax credit, depending on which works best for your situation. You may or may not owe tax in the US.
that's the simple answer - the specifics of forms, tax owed, exclusions, tax treaty position etc. will depend on your situation. And, if your foreign back account(s) had an aggregate value of $10K or more you are also subject to FBAR, which is reported to the Treasury Department, not filed with your tax return.