If you are already in the UK on any other visa you cannot cannot switch into this immigration category from within the UK. You will need to return to your country of origin and apply to the British diplomatic post there for ancestry visa if you:
♠ are a Commonwealth citizen intending to take or seek employment in the UK;
♠ are aged 17 or over; and
♠ can prove that you have a grandparent who was born in:
◊ the United Kingdom;
◊ the Channel Islands;
◊ the Isle of Man;
◊ on a British-registered ship or aircraft; or
◊ the Republic of Ireland if your grandparent was born there before 31 March 1922.
You can claim ancestry if either you or the relevant parent were adopted; or were born within or outside marriage in the UK. You cannot claim UK ancestry through step-parents.
If you are successful you will be granted five years’ permission to be in the UK. You will be able to bring your spouse, civil partner or unmarried partner and children with you or they can join you in the UK.