@Umami ********
Both the UK and US Embassies no longer provide income affidavits for their citizens. (Source UK and US Embassy websites)
The RTP IO are aware of this.
Other counties may still comply with this request hence it is still being referred to in the documentation presented.
As with all bureaucratic processes in any country it pays to prepare well in advance.
Most stress is self induced through lack of preparation and leaving everything to the last minute.
Smiling, admitting you've made a silly boo boo and the staff are more likely to help than send you away.
I use my UK Form P60 to verify my pension income with my IO.