Advice on writing a business plan
Firstly, read carefully any guidance you have been given for your assignment.
In the library, books about business plans are in the basement, at AA.BP.
Running this search in Google (not Google Scholar, just regular Google) will find guidance from UK universities and other academic sites:
You can also check for business plan workshops run by the British Library's Business & IP Centre. These are generally free, and take place at both the British Library and various locations around London. Some are also online. These workshops will be most relevant if you are contemplating launching a business, for instance based on your practice-based dissertation.