Bombing of the United Kingdom during WW2 can be split up into several overlapping phases:
The Battle of Britain began in early June 1940 with small scale nuisance raids on Britain. They were intended to train bomber crews, test defences and methods of attack. The initial intent was to destroy industry and air force targets; unnecessary loss of civilian lives was to be avoided.
By 13th August, this changed and Operation Eagle Attack (Unternehmen Adlerangriff) was the preparation for an invasion, commencing with the destruction of the RAF's fighters. The plan failed and on 7th September 1940, civilian areas were targets for terror bombing and the Blitz began. The frequent night time attacks continued, until 11th May 1941 when the German's attention switched to the invasion of Russia, which they had been planning for nearly a year.
There followed a period of relatively low intensity hit-and-run raids on coastal towns until the Baedeker raids commenced. These occurred in April and May 1942, with some targets selected as they received three stars in the Baedeker tourist guide books. Conurbations with a cultural value were targets until 1944.
The Baby Blitz occurred from January to May 1944 and was in response to the Allied bombing of Germany. Most of the the targets were in south east England, however the raids also extended to north east England, as far as Hull.
The V1 and V2 rocket attacks occurred from June 1944 until March 1945, generally targeting London and south east England.
In total, it has been estimated that 60-70,000 tonnes of bombs were dropped on the United Kingdom during WW2 and over 60,000 people were killed by the bomb & rocket attacks.
Every area of the United Kingdom was subject to bombing by the Luftwaffe during WW2, however some towns and cities were hit with a greater intensity of bombs than others. UXO risk assessments of construction sites at these locations should always be considered:
It should still be remembered that many other towns and cities were targets or victims of the Luftwaffe during WW2. The photo shows the bomb damage that occurred when a German bomber dumped its bombs in Rhyl, north Wales, when the plane was attempting to escape following a failed attack on Liverpool on 12th March 1941. The photo shows damage caused by one of only two bombs dropped on the town during WW2. Whilst it would be very unlikely that unexploded bombs remain in the area, it is still possible.
Both during and after WW2 significant efforts were made to clear unexploded bombs from the UK. Sometimes suspected unexploded bombs could not be located when excavation to find them took place. For most incidents, no evidence of a bomb was found and the report discredited. It was not feasible to recover a very small proportion of the bombs, generally because they were located in sewage works, cemeteries or in water logged ground. These bombs were abandoned and still pose a risk today. This media article describes the circumstances relating to an abandoned bomb at the 2012 Olympic site.
A camouflet is an underground hole formed when a bomb explodes at depth, without making a crater. For a 500kg bomb, a camouflet with a diameter of 6 metres could be formed. Whilst unlikely, it is possible that camouflets could still exist.