22301 implementation with scope of IT department only

First is important to note that certifying only IT department is very uncommon for ISO 22301, because in general it does not represent any business core activities (i.e., it does not deliver products and/or services to business customers).

To go for this approach of certifying only the IT department, you need to take into account all the services it provides as an IT department - to both internal and external users. 

So, in your BIA you need to consider all products and services provided to both internal and external users.

These articles can provide some tips about performing BIA:
- How to implement business impact analysis (BIA) according to ISO 22301 https://advisera.com/27001academy/knowledgebase/how-to-implement-business-impact-analysis-bia-according-to-iso-22301/
- Five Tips for Successful Business Impact Analysis https://advisera.com/27001academy/blog/2010/06/10/five-tips-for-successful-business-impact-analysis/

These materials will also help you regarding BIA:
- Implementing Business Impact Analysis according to ISO 22301 [free webinar on demand] https://advisera.com/27001academy/webinar/implementing-business-impact-analysis-according-to-iso-22301-free-webinar-on-demand/
- Book Becoming Resilient, The Definitive Guide to ISO 22301 Implementation https://advisera.com/books/becoming-resilient-the-definitive-guide-to-iso-22301-implementation/

Thank you, that is very informative and really helpful.