It is your responsibility as the driver to make sure you are insured to drive it, you're the one that will be prosecuted if you aren't.
Your company insurance is not going to cover you to drive a non-company vehicle on non-company business.
His insurance won't cover you to drive as you are not driving it on business related to his company.
Your friend should either contact his insurance company to specifically get you covered to drive that specific vehicle, or use a short term insurance like this.
http://www.aviva.co.uk/short-term-car-insuranc e/