For visits direct to client's premises, you have to determine whether or not the journey is principally an ordinary commuting journey - ie, is most of the route what you would normally travel on the way in to your employer's premises. For example, I live in Preston and used to work in Chorley, 17 miles away. If I visited a Client in Blackpool, 15 miles in the opposite direction, I would be able to claim 30 business miles. But if I drove to a client 15 miles past the office in Chorley, I would not be able to claim any! But if the client were 30 miles past Chorley, I would be able to claim 94 business miles (2 x (17 + 30)). Messy, huh? It was also complicated by my employer only allowing me to claim the lesser of the distance from home to the client and office to client. So I had to keep two different business mileage records - one for tax purposes and one for claiming expenses
That means that Rory's wife would be able to claim tax relief on journeys nearer than to the office (relief based on full 40p per mile tax allowance because no expenses paid) provided that she did not spend most of her journey on a route that she would normally take to work