Hi again Myra,
The reasoning is perplexing, but yes, you may end up reporting a total sum of funds twice the amount (or more) than you actually have due to the transfer of funds between current (or old and new) accounts. A parent may have savings in one account, then transfer that to a dedicated account (in the parent's name) to fund a child's university expenses, and the child may spend all the money in one year. For the IRS, the fact that there are no longer any funds in existence doesn't matter, and an FBAR would indicate twice the amount of (real) funds that ever existed in the first place. The same applies for the new (2011 tax year) Form 8938. If they are foreign assets and meet the 'aggregate' $10,000 or $50,000 levels for FBAR and 8938, they must be reported. Reporting on Form 8938 only begins for 2011, so there is no way anyone will be liable for late (or past) filings before April 15 of 2012 (8938 will be filed with 1040).
The calculation is very black and white: if the 'aggregate' results are over the respective amounts, they must be reported. If they are below the respective amounts, no reporting is required. (There is a caveat: 1040, Schedule B must be filed if you have any foreign accounts. If you have 'debit cards' for your UK accounts, they must be reported on your FBAR and 8938 as well. There are other 'assets' that must be reported, particularly on 8938.)
I can not comment on the surrender of your green card. Your best bet is the 'Britishexpat' forum (see Weller's post above). Recommendations on that site seem to favour Peter Newton, a US/UK tax advisor based in the US. Some of those who post on that site also post here, so maybe they could add more. The true expert on this site is guya. Although he is UK based, he may also be aware of the procedures. In the end, I would recommend direct communication with a truly international professional rather than comments posted by amateurs such as myself.