Business versus personal mileage

I have been supplied with a company vehicle (van), personal use is allowed and all personal mileage is to be reimbursed at HMRC guideline rates. I am based from home, and I either work from there, or visit customer sites throughout the country. I am required (usually once every 3 weeks or so) to attend the offices of my employer which is a 150 mile round trip. Does this constitute personal mileage for which I must reimburse my employer, or is it classified as business?

Hi

This would be classed as business.

thank you