1) interest is reported on Sch B, same as always.
2) you then take a tax credit for the UK tax paid on Form 1116.
3) It's easier with software than it is by hand. TurboTax is pretty good.
4) What happens if you don't use 1116?
a) you can take the foreign tax on Sch A instead. You will pay more tax using this method.
b) there's no law requiring you to take any tax deductions or credits at all. You can blithely go through life not taking a single deduction, credit, or exemption (other than the standard ones) and just pay tax on the entire kit and kaboodle if you prefer. Most people prefer to take the maximum amount of tax deductions, credits, and exemptions in order to pay the least possible amount to Uncle Sam.
c) therefore, the only thing that will happen to you is you will pay more tax. Uncle Sam will like that a lot; your bank balance won't.