We are currently relocating our stores warehouse into our main factory building, so there will be about four rows of ceiling height racking approximately 200M in length with fork lift access between.

I'm undecided whether or not additional fire exit signs are needed, what is the rule of thumb for this?

Before you start considering signage you need to look at means of escape, how far do people have to travel to get to a place of relative safety. Check out https://www.gov.uk/government/publications/fire-safety-risk-assessment-factories-and-warehouses.