GLP will be very good at explaining what you will be assessed against. Civil service have to tell you which competencies you will be assessed against and this will be relevant to the job grade you are applying to.
You can then see the type of evidence they will be looking for in the following document:
https://assets.publishing.service.g...e/436073/cscf_fulla4potrait_2013-2017_v2d.pdf
Just make sure you are looking at the right job grade.
Edit: - from their (awful) brochure
Making effective decisions – Using evidence and knowledge to support accurate, expert decisions and advice. Carefully considering alternative options, implications and risks of decisions. Communicating and influencing – Communicating purpose and direction with clarity, integrity and enthusiasm. Respecting the needs, responses and opinions of others. Working together – Forming effective partnerships and relationships with people from a range of diverse backgrounds, sharing information, resources and support. Delivering at pace – Delivering timely and quality results with focus and drive. Developing self and others – Focussing on continuous learning and development. Motivation – Having a clear interest in and motivation for the law, public service and the work of government lawyers.