Upvote:2
From the UK government website on the ancestry visa (bolding mine):
You can apply for a UK Ancestry visa if you:
- are a Commonwealth citizen
- are applying from outside the UK
- are able to prove that one of your grandparents was born in the UK
- are able and planning to work in the UK
- meet the other eligibility requirements
.
What you can and can’t do
You can:
- work
- study
- bring family members
You can’t:
- change (‘switch’) into this visa if you’re already in the UK on another visa
- get public funds
It doesn't appear that you have to wait until your current visa expires, but you do have to wait until you have returned to your home country