To qualify under the UK ancestry category for immigration, applicants need to be Commonwealth citizens who are aged 17 or over and able to show that one of their grandparents was born in the UK.
A common misconception held by people intending to apply for a UK ancestry visa is that they are required to have secured employment in the UK. Although this is preferable to enhance your prospects of succeeding with this application it is not a specific requirement of the relevant immigration rule. All that is required is that you can submit sufficient evidence to show that you are willing, able and likely to secure employment in the UK.
A UK ancestry visa is issued for a period of 5 years. At the end of 5 years in the UK on an ancestry visa you will be entitled to apply for permanent residence in the UK. 12 months after you are granted permanent residence you will become eligbile to apply for British citizenship.