A business plan needs to contain all the information that somebody would need in order to share your vision and also to understand the financial viability of the project. In particular your “break-even” point, best and and worst case scenarios.
A business plan often accompanies a finance application or is part of a proposal to potential investors. The plan could be read by people with a wide array of business skills and it is important therefore to make it as easy to understand as possible.
The Princes Trust offers a wealth of free information that would serve as a useful starting point
https://www.princes-trust.org.uk/help-for-young-people/tools-resources/business-tools/business-plans
Or you could try a subscription service that was recently recommended to me by a regional business manager of a large national bank (we have not used this product at Darrall & Co so cannot endorse it specifically, but the reports derived from this service, that we have been shown, did look impressive):
https://www.liveplan.com/features/impressive_plans
For a wider read (that will bring you back to the Princes Trust resources) you could try this link: https://www.gov.uk/write-business-plan
If you would like some help throughout the process of writing a business plan please get in touch: