WoW, if there is a thread likely to invoke an instance of Godwin's law, this is it!
Nobody can seriously contest that the BoB was an important... well, 'turning point' in WWII. However, even if the battle had been lost by Britain, all hope wouldn't have been gone. With Churchill, Britain certainly wouldn't have given up and German High Command did only half-heartedly prepare for an invasion of England - they were still expecting, pretty seriously, to make peace with Britain.
And, as this wouldn't materialize, Germany would have to invade, challenging the huge Royal Navy and the well-prepared, well-motivated Home Defence. On top of this, there would be inadequate equipment on the German side - river barges as troop transport which capsize in all but the calmest seas, plus the Luftwaffe had already expended nearly half their Ju52 fleet over the Netherlands, so airborne assault troops would be hampered as well.
Additionally, as Churchill stated in his "we will fight on the beaches", at least part of the Royal navy would have carried on "beyond the seas".