Let us begin at the point at which you have just taken possession of your hire car. There are now two things that you need to know - where you are going and how to get there. This brings us on to maps!
Ordnance Survey produce a map 'OS Travel Map - Route' which covers the whole of Britain with half of the country on one side of the sheet and the other half on the other side. This is useful for long distance planning and route finding and the scale is about 10 miles to the inch.
The ideal maps for exploring are the Ordnance Survey 'Landranger' maps. These are on a much larger scale of about 1.25 inches to the mile and show every little road. Motorways are marked in blue, 'A' class roads in red, the 'B' class roads in brown and all other unclassified roads in yellow. Any white roads shown are often just unsurfaced tracks and may be private. Each sheet is about 3 feet square, folded in half first and then folded in concertina fashion so that it is possible to look at sections of the map without unfolding the whole sheet, and covers an area of about 28 miles square. I feel totally lost if I go anywhere without these maps. You can see a sample below.
The yellow roads are unclassified roads (back roads) and the Brown road, bottom left near Clifton upon Dunsmore, is a 'B' class road.The white 'P' on a Blue square, top left, denotes a car park and next to it the 'X' with a bar across the top, which is meant to represent a table, denotes a picnic area.
You could probably find your way easily on the major routes by following road signs but if you want to explore the unclassified back roads (you will miss a lot if you don't) do not rely on road signs. I have frequently arrived at a 'T' junction on a back road to find that there is no sign at all!
You could try the Automobile Association's Route Planner. All you need to do is enter your starting point and destination together with any locations that you want to pass through and it will give you the total mileage, estimated driving time and navigation instructions.