*** Moved from Bringing Family of USC forum to IR1/CR1 Process & Procedures, where topics about I-864 for spouse of USC are discussed ***
As a US citizen, you are required to file US taxes, regardless of where in the world you live. As a married person filing taxes separately, if you earned income more than $5 (yes, five, not 5k) in a year, you are required to file taxes. For your family to immigrate to the US, you will be required to submit your US tax transcripts or tax returns for the 3 most recent tax years. I suggest you look for a competent tax professional who handles tax filings for expats, to help you back-file your US taxes. You will also need to report your UK or other non-US bank/investments accounts via FBAR filing. If you hire a tax professional, make sure they are familiar with the IRS amnesty program for expats, so you can minimize the penalties for late filing.