Thank you for your question and patience, I’m Tom and I’ll try to help you.
Once your application has been approved you will received a certitificate of naturalisation. You must then book a citizenship ceremony with your certificate.
At your citizenship ceremony you will swear allegiance to the crown.
Once you have done this you must apply for your UK passport. Once you have your UK passport you are free to travel to the UK and settle here.
Once you have entered the UK you can then apply for a National Insurance Number in order that you can start working. I do not believe you can apply for one given your current circumstances and must wait until you receive your passport.
There is no such thing as a security number; I suspect you were referring to the national insurance number.
You can attempt to look for work now but I would make it clear that you await your UK passport as proof of your right to work. Likely, UK employers will postpone offering you a job until such time as you have obtained your UK passport unfortunately.
My goal is to provide you with a good service. If you feel you have received anything less, please reply back as I am happy to address follow-up issues specifically relating to your question.