For doing the underfloor, engine bay and all underfloor components right its easy to spend more then 350+ hours of labour.
Dismantling. Blasting. Priming. painting / powdercoating (avoid where you can). Assembling. Using seam sealer and spraying rubber on the underfloor.
I didnt work with POR15 so i dont know how hard it is to remove, but some additional hours may come removing it compared to a car that didnt have this treatment.
The firm will have to spend 1000GBP or more materials used and services outside for blasting, plating etc..
More or less you will know everything from the quote given and hours listed.