Unfortunately our world is not flat as believed in the middle ages, but the earth is a sphere (or more exact a geoid).
On a sphere you can find any point by providing 2 angles and 2 reference lines. The reference lines are the equator and the 0 meridian (a line running through Greenwich and the 2 poles).
The latitude is measured as an angle from the equator and is given in degrees north and south (max 90°). Each degree is split into 60 minutes. E.g. Our clubhouse is at a latitude of: N 55° 9.99min
The longitude is measured as an angle to the 0 meridian and is given in degrees East and West (max 180°). E.g. our clubhouse is at a longitude of: W 1° 34.23 min
By definition one nautical mile is 1° of latitude. As the circles get smaller the closer you get to the poles this is not true for the longitude (except on the equator).
For the mathematically minded: 1° longitude = cos(latitude) nautical miles
so at our latitude (55°N) 1° of longitude would be: cos(55°)nm = 0.57nm (please check this on your charts)
The OS Grid is based on a modified Mercator projection of the British Isles.
In a Mercator projection the sphere of the earth is stretched at the poles to fit on a cylinder.
Basically a grid was put over the UK and everything was stretched to be flat, resulting in areas in the north being more stretched than in the south. Equally, vertical lines on an OS map don't necessarily point to the north. So there is a difference between grid north and true north. (e.g.: on the OS Landranger map 81: in the NW corner grid north is 0°04'E, at the NE corner it is 0°27' ....)
Positions are read on an OS map by first identifying a 100.000m square. This is for us NZ
then you read the eastings and then the northings. So once again for our clubhouse the OS coordinates would be: NZ 274 858
Measuring distance and speed:
Originally the word miles comes from the Romans and stands for 'mille passuum' (a thousand paces) in 1592 under the reign of Elisabeth I the statute mile was defined. 1 mile = 1,760 yards = 5,280 feet. this is still used in England for measuring distance and speed (in miles per hour) the nautical mile is based on the diameter of the earth and is 1 / 21600th of the earths diameter, or easier 1 minute of latitude. A speed of 1 nautical mile per hour is called 1 knot.
1889 the International Bureau of Weights and Measures in France defined the meter as a unit for distances. 1000m equal 1km.
1 nautical mile = 1.15 statute miles = 1852 meters