Definitely file an amended return for 2010 if your original return was missing anything at all (even if it's just a small change, I'd do it for the peace of mind).
You'll need forms 1040X, 1040, and Schedule B. Also check if your changes affect any of the other forms you initially filed, and include them only if they have changed. For instance, your Schedule B isn't likely to change anything on your 2555. If that's the case don't re-file the 2555.
No need for a covering letter - Form 1040X includes a space for you to explain the changes you are making.
No tax is levied on the balance of accounts declared on the FBAR. The government just wants to know the balance so they can fight money laundering and the like. Of course you should have been paying tax on any interest you are earning in the UK; if not, pay that tax on your amended return.
If you have not filed FBARs for previous years, file them now and attach a cover letter saying you were unaware of the requirement to file. The IRS says: "If you learn you were required to file FBARs for earlier years, you should file the delinquent FBAR reports and attach a statement explaining why the reports are filed late. No penalty will be asserted if the IRS determines that the late filings were due to reasonable cause. Keep copies of what you send for your records." (from
http://www.irs.gov/businesses/small/article/0,,id=210244,00.html#FR16)
If you should have sent Schedule B for previous years but didn't, I'd suggest you amend those returns as well (meaning send 1040X, 1040, Schedule B, and anything else that changed).