Hi KP
As the warehouse isnt sharing the dno supply with a domestic property you do not need to worry about part P notification, the work will be classed as commercial/industrial.
The legal side of things boils down to your competency to undertake the work, so if you work & show you've worked to EAWR 1989 & BS7671 & have a working knowledge of what you plan to do, then you have no worries (nothing ventured nothing gained), but I would point out that as a DI you may find you will be working on a three phase system in the ware house & I dont know if this is covered on the di courses with regard to testing .
All electrical work should be carried out to comply with BS7671 including the testing.
As for certificates go to the IET site and down load them free or photo copy them from the back of BS7671.
The ones from the IET can be filled in on the pc via the adobe download.
Heres a link to the certs
Forms for electrical contractors - The IET