Agree with Kezzie62, US SS payments to a UK bank should be no problem. I also have US SS deposited into a UK account. SS payments are eventually made from CITIBANK, Ireland (according to my statements) from the SSA to my account with a UK high street bank.
As for US government, US company, or other US pension providers, I have no idea. US Government or company pension providers may require a 'SWIFT' number.
If you have pensions other than US SS, you may wish to consider opening an additional current account at another bank. I have a 'SWIFT' number for HSBC but don't know whether they still use the SWIFT system as well as IBAN. It's the SWIFT number for the old 'Midlands Bank' network and starts with the letters MID_ _ . You might give HSBC ( or Lloyds or Barclays) a call if the US provider can only handle a SWIFT number.