In light of similar recent topics, let's start from the top.
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Your husband is a US citizen. He has a right to a US passport, and may use it to travel to the US or elsewhere. When he applies for a new passport he will now have to include his US Social Security Number (SSN), the same number used for tax identification.
The confusion: There are laws proposed, laws currently on the books, and laws in people's imaginations. Is the IRS ruthlessly combing Dept. of State records looking for the SSN’s of USC's living abroad who are delinquent with tax and asset reporting? The IRS or Treasury may request info from the DoS, and there may soon come a time when the transfer of info (SSN) is automatic. Some believe it already exists. Right now, it is difficult for the IRS to interfere with a passport application or have a passport confiscated. They may be able to do it, but it takes a judicial process and a threshold ($50,000??? of tax due???proposed???) before any action can occur. The IRS can confiscate assets without notice, and it does happen, but for someone abroad with no US assets, it's rare unless large amounts are being investigated. Scaremongering is running amuck, but there is a basis for contemplation. Right now, it's unlikely for the average individual abroad. Next week, next month, next year, who knows? And, once you have arrived in the States with passport in hand, the IRS knows where to find you easily.
If your husband's yearly income is above the relevant filing threshold for his status (MFS or MFJ), then he is required to file a US tax return. For 2014 tax year, the MFS (married filing separate) filing threshold was $3,950. So yes, at £18,000/year, if filing MFS a return is required. For years in the past, the thresholds were slightly less.
If your husband wants to become compliant (and he may not want to, it’s his choice to be non-compliant, but there is a substantial risk involved if non-filing continues. YMMV), then the
Streamlined foreign offshore procedures would be the recommended programme. The following gives all information on how to proceed. You can complete this procedure yourself without seeking professional assistance if you’re confident of your actions.
http://www.irs.gov/Individuals/International-Taxpayers/U-S-Taxpayers-Residing-Outside-the-United-StatesAs regards FBARs, the procedure requires FBARs for the past 6 years, but if he was under the threshold (aggregate of $10,000), there is nothing to file. Nonetheless, it may be prudent to include the last 6 years even if the amounts are very small and below the threshold.