If you do agree to an interview, you should do so under strictly agreed conditions with full pre-interview disclosure. Ideally, you can simply decline the offer of an interview and say that you will supply written answers to any questions that HMRC wishes to submit to your professional adviser in writing.
Although HMRC will claim that this is your opportunity to show that you have not committed tax fraud, in reality, HMRC would not have proceeded this far and devoted such resources to the investigation without a strong belief that you are guilty.