There are many reasons NOT to consider MFJ, the main one being that the income of the non-US citizen spouse must be included. The typically ends up with a higher tax liability than might otherwise be the case.
If the non-US citizen spouse has NO income from US sources (including interest or any other type of income) then the US citizen spouse can claim an exemption as part of the MFS (which is the way the return would be filed). This requires obtaining an ITIN for the non-citizen. Formerly you could just write "NRA Spouse" on the line that asks for the social security number, but the rules on that changed a while back.
Your wife's requirement to file a US return kicks in at any point where her income exceeds the MFS threshold, which was $3900 for 2013. Trying to claim that you made <$20K and would have filed a joint return, thereby eliminating the need to file won't cut it with the IRS. In any case, unless your wife makes megabucks, given that the UK tax rates are higher, it's doubtful that she would owe any US tax.