Is 2mm wire fine for adding additional wiring on a 1970 series 2a 109 ex military, thanks
You need to be more specific than that. For a start, for what purpose...?
AES catalogue gives typical usages and relevant sizes .https://www.autoelectricsupplies.co.uk/file/aes-catalogue-issue-11-website-version-1.pdf
It was just for general items like maybe temperature gauge, sorting some broken wires etc
There aren't many wires on a Series 2 with a core more than 2mm. I'd say most things that aren't connected to either battery charging or engine starting will most likely be fine.
https://www.autosparks.co.uk/frequently-asked-questions
if you go here and then to the bottom of the page, to "cable application guide". it gives you a good idea of what to use