Hi, we get tax credits and DH is self employed. I don;t think you would lose it by going self employed, as it is based on your profit. So, if in first year it was low they would just add that amount to your Dh's income to calculate your total income and base it on that. You can even do something were if you make a loss you can take that figure from the total income also. If you have a look online at tax credits and self employed it explains it.
Perhaps you mean it could be a change in circumstances leading to a new claim for UC instead? You could contact tax credits and check that out- there helpline and online chat is pretty useful.