All UK Motorways and some dual carriageways have marker posts placed on the nearside edge of the carriageway usually at 10m intervals. These are white poles, usually about 1m high, white with a red reflective vertical bar on the face and a blue band running around the top. In the blue segment of the post are some numbers. These numbers show the distance to/from the start of that particular road or motorway in kilometres and metres.
Most motorways start at London, (M1, M4, M3) so that the marker posts can be used to tell which direction you are travelling in; towards or away from London. Some other motorways branch off from other motorways, however, (M5, M69), and the posts on these count up as you drive away from their source.
The junctions on the motorway also increase as you travel from the source, so that, by observing the numbers on the marker posts from one junction you can determine what the number of the next junction will be.
Marker posts also indicate the direction to the nearest emergency telephone, with a 'phone' symbol on the side of the post and an arrow. The posts can also be used to locate a specific point on the motorway for emergency services, breakdown recovery, etc. The posts will not show which side of the motorway you are on though (Northbound, Southbound, etc) so for this you must check as above and consult your map.
Because they are spaced out at regular intervals, the posts can be used to accurately check your speed if you want to calibrate the speedometer in your vehicle.