http://www.raf.mod.uk/history/backgroundtothebattleofbritain.cfmOn
This brief RAF history of the Battle of Britain lists four phases, one
of which is the Blitz. From their point of view the BofB was the defence
of Britain from attack by enemy aircraft[1] and thereby making a
sea-borne invasion unfeasible:
http://www.raf.mod.uk/history/backgroundtothebattleofbritain.cfm
Phase 1 - The Battle Begins
....
Starting on 10 July 1940, the Luftwaffe attacked shipping convoys in
the Channel and Channel Ports.
....
In this stage of the battle, the Luftwaffe was in effect probing the
British defences - looking for weaknesses before a major assault
could be launched to exploit them.
Phase 2-Pressure grows
At the beginning of August, with German invasion forces and troop
barges being assembled on the French coast, the raids against the
South coast of England were increased in size and number.
Believing that the British early warning system had been destroyed
and the coastal towns sufficiently 'softened up' for an invasion,
the Luftwaffe began the next stage of their plan.
On 13 August (called Adlertag or Eagle Day by the German High
Command), massive raids began on the airfields of 11 Group. The aim
was to destroy the RAF, either in the air or on the ground, in South
East England. To put pressure on the British defences, the Germans
sent high and low level raids to different targets at the same time.
....
However, just when it seemed that the country and 11 Group in
particular couldn't continue for another day, the Germans changed
their tactics.
Phase 3 - The Blitz
....
By attacking cities and industry, the Germans hoped to break British
morale and to destroy the factories that built fighter aircraft.
They also hoped that RAF fighters would gather in force round the
cities to protect them, which would make it easier for the Luftwaffe
to shoot them down in the numbers required to establish air
superiority.
The change of plan was a mistake for a number of reasons. It gave 11
Group a chance to repair their airfields and radar sites, so the
defences became fully operational again. The German Me 109 fighter
could only carry enough fuel for 20 minutes flight over Britain, so
London was on the edge of its limited range.
....
For the people living in the cities, the Blitz had begun, as night
raids followed daytime raids and gave the civilians little rest.
Everybody was in the front line, and there was little the RAF could
do to stop the night raids. Airborne radar was in its infancy, but
there were some successes for the Blenheim, Defiant and early
Beaufighter night-fighter Squadrons. Some of the Hurricane and
Spitfire day-fighter Squadrons also took part in the night defences,
but relied largely on luck to make an interception.
Phase 4 - The End of the Battle
As the long, hot summer ran into October, the German daylight bomber
losses became too heavy. Their bomber force started to operate only
at night, and the damage they caused to Britain's cities was
enormous. Many civilian organisations were set up to help deal with
the wounded people and damaged buildings.
The German raids continued, but the RAF had started to develop night
fighters equipped with radar which could tackle the problem.
....
The Germans then realised that the RAF could not be defeated in
1940. Germany was also preparing to attack Russia, so Operation
Sea-Lion [the invasion of Britain] was cancelled indefinitely and
eventually abandoned altogether. The Battle of Britain was over.
Strangely, for such a ground breaking Battle, the first to be
decided purely in the air and the first real test of air power as a
defensive and offensive weapon, it did not really end, so much as
petered out.
[1] The V-1 cruise missiles ("flying bombs" or "doodlebugs") and the V-2
ballistic missiles were not used by the Germans until 1944.