Claim the business proportion, and be able to explain how you arrived at it. A phone used 70% for work means 70% of the bill is allowable; a van used privately at weekends needs a mileage-based restriction. The method matters less than the evidence: a defensible basis you applied consistently will survive scrutiny, while a round 50% you cannot justify will not. Working from home follows the same logic, though there is a flat rate available that avoids the calculation entirely and is usually the right answer for modest use. What HMRC objects to is not apportionment but the absence of any reasoning behind it. Record the basis when you set the split, not two years later when you are being asked to remember it.