I couldn't find any photos of the connector that weren't drawn, but it looks like a fairly standard 2-conductor wire. The pinout is what matters, and due to the voltages being used would not suggest buying anything other than via official sources.
The support contact info can be found on the page I look above.
the pinout and connector type can be seen in https://nest.com/support/images/misc-assets/nest-protect-install-guide-wiring-and-battery/nest-protect-ill-uk-nest-bp.png
Hai bro... here you can build your own cable connector for Nest Protect ... folow this link ---> https://www.studiopieters.nl/nest-protect-second-generation/ perhaps this site can help you.....
moste likely we'll need to have more informations about the connector type and what's used for please;then we'll look for all the parts needed to this nest item,pins can be the key to find the connector
the connector is for three pins and can be seen in the following figure https://nest.com/support/images/misc-assets/nest-protect-install-guide-wiring-and-battery/nest-protect-ill-uk-nest-bp.png
Regarding the pinout answer below, this is not strictly a problem AC electricity will work with live/neutral reversed, but it does have safety implications as the wrong (neutral side) might end up being the fused side. Generally speaking as long as the feed is RCD protected back at the main board, and the unit that is being wired to is 100% class 2 insulated (no exposed or accessible metal) then it may not really matter which way round. However I would try calling 0808 169 2307 which is the Nest helpline who may be able to send a spare cable assembly out to you.
After talking to nest support it seems they should be able to supply a replacement cable provided you have access to the product. I attempted to get the information myself but they asked me to supply a serial number that I obviously had no access to. Sadly the best answer that I can provide is to contact nest support yourself and provide the information they ask. https://nest.com/uk/support/contact-us/