It is a good what if;
At a crucial juncture in the Battle of Lexington and Concord, a substantial Massachusetts Militia force from Salem and Marblehead under Colonel Timothy Pickering arrived in the vicinity and had an opportunity to block Lord Percy's route into Charlestown and safety by seizing the road over the neck of the peninsula. He was later accused of not doing so due to his own opinions about a peaceful resolution of the conflict. What if he had moved with more pace and cut off the retreat? Would the expedition be annihilated? Would this allow for a storming of Boston?
From what I've read, there was certainly a possibility to cause a lot more casualties to the British column on the retreat than even what occurred historically; given the fatigue factor and ammunition, it's probably an even bet as to whether the British would be able to crash through a blocking force, disperse, surrender, or all of the above.
I don't know how much of a garrison the British had left in Boston, and how much naval support they had; likewise, given the general ad hoc nature of the American force, hard to see who would have had the authority to order an immediate assault. Presumably Warren, but I'd expect he'd need at least some of the artillery from Ticonderoga.
If the initial battle/campaign of the conflict turns into an outright British defeat - as opposed to a withdrawal - then one can only think there might be even less willingness by St. Germain or whoever to mobilize and start sending expeditionary forces 3,000 miles across the Atlantic.
Possible ripples could run from a negotiated settlement in 1775 to a rougher war for the British if they prolong the conflict to less of a sense of shared nationhood among the Americans and balkanization to an absolute collapse of British control over all the thirteen colonies, plus the Canadas and maritimes as an incipient confederacy, with what all the above means to the future of North America; push it far enough, and there's the question of Ireland and Scotland, which if it leads to independence, has all sorts of ripples in Europe...
It is certainly an interesting potential point of departure...
Best,