It was one of the strongest and powerful navies in the world.They could keep using ammo without fear of them running out.They didn't experience what the colonist did because they always had extra supplies.The British were VERY wealthy.
They had generals and soldiers who had experience.They were able to practice and form routines for certain aspects.British often outnumbered the colonists when they had battle.
Distance between England
From England !!
The citizens were able to contribute and have their own say in a lot of things that were done.This protected the British citizens from tyranny and stuff like that.
The British soldiers were fighting in unrecognizable territory.If there was any shortcuts they wouldn't be aware because this is land that some of them never even came to before this occasion.
Since the British soldiers were in the colonies region they were very far from Britain.It took long for more soldiers,ammunition,or anything else they needed to arrive.It was also very expensive to get back and forth.
The British citizens were never really convinced to support the war.Money was being wasted and they were worried about risking their lives.The British did not expect this war to happen for this long and alternately became tired.