In the United States this is known as travel and expenses and often abbreviated T&E. As Michael pointed out, you want to fully disclose your policies up front so that there will be no surprises later.
You may want to use something like this:
Travel time will be billed at the rate of $50 per hour and the client
will be charge for all related expenses including but not limited to
mileage at the rate of $0.51 per mile as well as all tolls and parking fees. Should
overnight travel be required all meals and hotel expenses will be the
responsibility of the client and the travel time expense shall be
capped at $400 per day.
In your initial conversation, email or whatever, state that you have a call-out fee and let them know what that fee is, including any travel expenses, for example:
"I have a fixed call-out fee for your location of £80. This includes
the first 30 minutes work and fuel expenses"
You can say, "So there are no surprises for you later, I would like to let you know that there are associated charges with coming to you. I hope this is okay."
In my business, we capture this in a stated one-hour minimum, portal-to-portal. From the time I step through the door, our billable hours start. If we walk in, rectify the issue in 2 minutes and are back out the door, we bill for an hour. If it takes us an hour, our hourly rate is enough to cover the travel costs.
Ideally, you would not want to cover a huge geographical area like this, but I put a hard limit on customers being within a 20-mile radius of the office. It is rare that we get a request outside of those geographic confines, because we do not market outside of that. If we do, we charge from the time we leave until we either get back to the office or to the next job.
If you state it upfront, politely but firmly, no one should have a problem with it. Enforce the rule across the board, to everyone. If anyone expects you to come to them for free, you do not want them as a customer anyway.