The US tax system requires US citizens to file each year. You need to file at least 3 years of US returns.
For US tax purposes you need to report worldwide income and gains, including employer contributions to UK pension plans and other items that would be tax-free in the UK such as UK child benefit, interest and dividends in an ISA and premium bond prizes. If you have invested in a UK unit trust or investment trust you will be subject to penal rates of tax.
Up to $80,000 worth of foreign earned incomed can be excluded each year.
In addition to filing income tax returns in the US you need to file a form TDF90-22.1 each year if the balance in your UK bank account exceeded the equivalent of $10,000 (even for a day). These now carry an automatic $10,000 penalty for late filing.
As you and your spouse have different domiciles you may also need to consider inheritance and estate tax planning as you cannot leave things to each other free of tax if either of you die.