In April 1918, the Royal Flying Corps and Royal Naval Air Service amalgamated to form the Royal Air Force. The new service occupied 301 airfields, including airship & fighter stations, and training depots. Once WW1 ended, only 30 were retained.
In the build up to WW2, 100 permanent airfields were built and by the start of the war, once civilian sites were requisitioned, there were about 270 airfields. Many temporary airfields were constructed during the the war, most having paved runways. By 1945, the UK was described as one vast aircraft carrier anchored off the north-west coast of Europe, with 720 airfields (including flying boat bases) and 9000 miles of runway. By 1945 the UK's airfields covered 360,000 acres, an area larger than Bedfordshire.
The three significant potential sources of UXO to consider on former airfields are: