EIM32231 - Travel expenses: travel for necessary attendance: passing work on the way to somewhere else: example
An employee drives each day between his home in Southampton and his office in Winchester. One day he has to travel on business to Birmingham. They travel directly from home to Birmingham but stops off at their office to collect some papers.
They stop in Winchester is incidental to their business journey to Birmingham. Therefore the whole of the travel between Southampton and Birmingham is treated as a single journey to a temporary workplace, see EIM32075. A deduction is available for mileage allowance relief, see EIM31200