3 years proof of residence

[Laurenward]

I am a British citizen by decent (my father was born in Scotland). I lived and worked in the UK for 4 years (with some holidays during that time). I now live in Zimbabwe.

I am trying to apply for my childrens British citizenship but need proof that I lived in the Uk for 3 years. What documents can I use. Does a letter from my employer count? Do they have when I entered and left the Uk during this time period on their passport database to make sure I have the required number of days living in the Uk.

Please advise. Thank you