6-7 years ago, EU has harmonized a process of customs documentation(s) and has made it digital. A Common "E-System" was introduced. In the UK it was NES (National Export System) and I can imagine it was not dropped after Brexit and you (UK) still use it.
Point is - all forms are digital now, "E-System" will generate the latest correct version of the form and this is probably the reason why gov.uk hasn't update C88 chapter since 2014.
Are you Business or Private individual?
I do ask because you need EORI number to work with this SAD (aka C88) and C1314 forms. Sure you can register, but typically only businesses who are doing regular exports apply for the EORI registration (and access to NES).
If you are private individual, I guess you will not self organize transport outside UK on your behalf - you will use RM, UPS or any other carrier - anyway, you will use 3rd party company to assist you in the exportation.
A quick check on RM website and looks like you need to fill CN22 or CN23 "stickers"
https://personal.help.royalmail.com.../~/help-with-customs-and-sending-items-abroad
Those CN22&23 are names reserved for postal service, so DHL/UPS may have it named differently but for sure these couries have something like customs declaration.
https://www.gov.uk/bfpo/customs-declarations
I think this is all you need to do as private individual, you need to fill "customs declaration form" given by the carrier and they will submit C88 and any other forms into NES.
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