Two rates matter for fleets. Heavy vehicles travelling on public roads get the excise rate minus the Road User Charge. Off-road use, and auxiliary equipment like fridge units or agitators, get the full rate, which is higher. Light vehicles on public roads get nothing.
Rates change at least twice a year and apply by the date the fuel was acquired, which is why claims need dated fuel records and a defensible on-road and off-road split. The fuel tax credits guide explains the method.
General information, not tax or legal advice.