I know of US citizens in UK who did not file US taxes for more then 20 years.. That said, US IRS is more and more after its citizens abroad (and in general, there is more and more of information sharing between countries etc.). So if you do not comply, your chances to get caught are not small.
However, UK tax rates are higher then US tax rates (and you can count UK taxes paid against potential US ones), and US citizens living abroad do not pay taxes at all from first about $100,000 of earned income (i.e. salary), so most probably your wife would not owe any actual tax (unless she heavily engages in passive rental activities and UK stock market)
The important thing though is that if she just files backward US taxes and FBAR now, IRS might go after her with absurd penalties.
They do have a protected route of filing "delinquent" returns under some no-penalty or minimal penalty guarantees, but one has to be very careful to make sure it is done correctly (so that they would not go after her).
There is a forum of US expats in UK (something like "UK-Yankee"), they might help, though it is not very populated.
IRS website itself might be very useful, if you are obsessive enough to actually read it.
And there are plenty US tax lawyers in London - it is probably worth it to pay them once for consult.
There is also an option to renounce US citizenship..