For your business plan, HMRC are looking for whatever information would be presented to potential investors to entice them to invest in the business.
So, generally speaking, whatever you have already prepared is usually the best starting point.
We'll be sure to help you ensure the business plan is clear and concise as part of our Advance Assurance review. However, to be sure you're prepared, check out our full content guide for the Advance Assurance application business plans.
In addition, HMRC have made some business plan templates available.