Sorry to disappoint, but it's not an anunual interest rate, rather it's an agreed interest rate, which has been agreed between HMRC and banks as roughly the interest a customer would have received from having the PPI cost in a bank account for the term of the loan. This is as follows
You get a repayment back in three parts:
1. Refund of the actual payment protection insurance payments made
2. Refund of associated interest on the premiums
3. Payment of 8% statutory interest
The 8% statutory interest is the only part that is taxable. The reason you are repaid this interest is because this is what that money would have earned in a savings account had you not spent it on the policy. You would have paid tax on it anyway and in comparison to the full amount owed to you it should be a small amount.