I am currently staying in the UK under a work visa. My parents, who have never been out of the United States, are coming to visit me and want to see all that the UK has to offer. I have only traveled by the train since I have been here, since the company has bought me a Train pass for my commute. I was wanting to know what is the most cost/benefit of using a Train vs a Car?
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The main cost of travelling by car are the cost associated with owning or renting a car. As you do not own a car, you will have to calculate with the rental costs.
In the UK, the main factor in the cost of train tickets is whether you book in advance and accept that you are restricted to just one train or want the freedom to decide on the day and buy your ticket last minute. A rail pass might help, but do check that those are cheaper than your planned journeys, when bought at the time you will. Here is a link to the site of the man in seat sixty-one on BritRail There is a lot more information on train travel, in the UK and farther away, on that site. If you go for the 'buy train tickets' route, look into a 'Two Together Railcard for any age' or an age related card if they qualify for that. Also see the comments on the question, the price quoted is £30 for the two together card.
The actual cost depend on those as well as on your actual travel plans. So it is not possible to predict which method of travel is cheapest.
Instead of using just one mode or transport I would suggest a combination, using trains for long distances and booking early to get the best prices. And when you are in an area you want to explore better, either use local trains, which do not have the early booking discounts, rent a car for a couple of days or join with a local tour company that shows you around where you may not want to drive.
That combination allow you to cut on train cost, like have a train ticket London-Edinburgh booked as soon as you are sure they will be with you that date, and allows your dedicated driver relax on the long distances.
If non of you have driven in the UK/on the left before, it might be that non of you feel happy driving small country lanes, even less on mountain roads.
In that case I think local tour companies might be the way to explore.
When happy to drive wherever, a rental car for one or two days at a time might be better.
But however you organize your trip, make sure you have enough of time, more than tourist tend to plan. It is no fun to be on the move every day, having to drive long distances to your evening accommodation or riding trains most of the day to do an other long distance the next day again.
One extra point against train travel, you should not bring too much luggage, one small to medium case per person and a small bit of hand luggage will be best.
A last option is to book a tour showing the best of Britain, all by coach, which will be the best option if they are short on time and still want to see a lot.