Maybe the OAP could do a version of this post from that perspective.
Whoa! This thread is going in all directions. A lot of statements are being made, and I'm not sure I agree with all of them.
Do you want the OAP's version of the method of reporting a store front tax service in Keokuk, Iowa would use, or a US tax specialist in a foreign country would use, or the method an IRS agent(s) would suggest (reserving the right to invalidate that suggestion at a later date)? I assure you, the conclusions they reach can be different. Given the lack of explicit directions, this does not surprise me. In fact, the lack of directions may be a clue. Or, do you want the method of reporting a self preparing Yank sitting in the middle of the UK with limited knowledge would use.
Form SSA-1099. You receive a copy. The IRS receives a copy. Box 3 lists the (gross) benefit paid for the year. Box 4 lists benefits repaid to SSA (mine says NONE.). There is space for a description of the amounts withheld in Box 4. Box 5 lists the net benefits. Box 6 lists the Voluntary Federal Income Tax Withheld. If you live in the UK, have your SSA check deposited into a UK account, and have no voluntary withholdings, Box 6 says NONE. There is no box that is devoted only to US federal tax withheld.
Before I start, I concede that the following will be deemed wholly incorrect by others, and will lead to the End of the World as we know it, and it will all be my fault.
After assimilating the information from all those listed in the 2nd paragraph, that self preparing Yank in the Cotswolds may do the following:
They would list the amount in Box 5 on line 20a of Form 1040. They would enter '0' on line 20b. Above line 20b, in pencil, they would note the SSA income is exempt under the US/UK treaty, Article 17. They would prepare 1040 in this way on the specific instructions of several IRS agents. They have recorded names, badge numbers, times/dates, information given, and message numbers if appropriate.
If they wish to invoke the treaty, (the SSA is not taxable by the US), they do not mention the SSA income anywhere else on their return. They may complete a Form 8833, if they wish, but according to the IRS agents, it's not obligatory for exempting SSA. They can file 8833 just to confirm the treatment of the SSA income, if the return is questioned. They do.
The Yank in the Cotswolds has multiple foreign pensions, including the UK State pension. They do not exempt the UK State pension from US tax. According to very specific instructions from IRS agents, they DO NOT enter these on line 16a, regardless of instructions in Publications stating otherwise. Instead, after deducting their allowable costs towards the pensions by use of Publication 939, they enter the total net amount on line 21, and label them 'foreign pensions'.
For Form 1116, they do not include the UK tax paid on the SSA income. They do this by apportioning the amount taxable by the UK of total pensions received (not the total of the pensions) to the (UK taxable) amount of the SSA income, and multiply total tax paid on pensions to the UK by the result. The stance taken is that if the SSA is exempt from US tax, then the UK tax paid on the amount would also not be allowed. This conclusion MAY NOT BE CORRECT, but in the case of the Yank, it makes no difference to the final US tax bill, and only results in reduced excess credits for carryover/carryback. They have tons already. Mileage may vary.
FINAL NOTE: The Yank in the Cotswolds has original recorded direct information from the IRS. You may not.
FINAL FINAL NOTE: I'm going to be in deep sh*t if there is a member here who uses the name 'Yank in the Cotswolds'. I haven't checked. If there is, this wasn't them.