Can anyone tell me what the white poles with an orange trianglular shaped roof, usually noted at the edge of fields are for.

At last, an easy question

All of the major RAF and USAF airbases around the UK, together with key installations such as AWE Aldermaston and AWE Burghfield, are connected to large oil refineries around the coastline using a massive network of underground high pressure pipelines and Petroleum Storage Depots (PSD), referred to as the Government Pipelines and Storage System (GPSS). For safety reasons, the locations of these pipelines are marked with identification posts which look rather like bird feeding tables with brightly coloured roofs.

The GPSS also connects into various commercial pipeline systems maintained by the likes of Esso, Conoco, TotalFinaElf, etc. and the network is fully described on a public website:-

http://www.linewatch.co.uk/network.htm

Check their 'Pipeline Markers' button for pic of your favourite 'Pole with a roof'.