For part p work Elecsa require you to have a guarantee available, you can arrange this yourself with an independent provider if you wish but Elecsa provide their own scheme for no cost, so why not?
You can only start this once you are through the assessment. If you are going to take it up with Elecsa then tell them at the assessment and you will be through that bit.
I would also say on reading the small print that if there is a Part P non compliance then whilst the guarantee will arrange rectification I believe they will then charge you for this rectification! as there is a section stating I agree to cover any costs associated with the repair.
Anyway no customer I have had has yet taken it out because it costs them more money.