You can drive at 17, HIRE COMPANY'S, dont allow hiring to people under 21, not the law or anything. You may be lucky and find a small independant who'll hire you a van with a larger premium.
Enterprise for example dont hire vans to people under 25, as my work uses Vans, buses, cars from them all the time, they waved this and allowed me to hire a Van at 24 with a small £5/day surcharge. They'll hire cars to people over 21 though.
As for your work, thats to do with their insurance. Your covered, you've passed your test, so you can drive.
Drives: Bravo HGT 155 Nov 2000 - Vroom Yellow
Family: Punto JTD ELX 2001 - Met Red - Volvo S40 2.0 TS 2002 - Red
Old: Brava 80sx 1999 - Red - Punto 60sx 1997 - Red