1550 too early ? NOT!!!
The English get J. Cabot in the 1st few years & S. Cabot not long thereafter. The coast of American may be well-explored--and colonised, as long as the settlers are available.
As you note, the 1st real English colony was at Jamestown in 1607. But, in my game, by 1608 I had colonised every coastal province from Bangor to Matagordos except for 4 provinces that Spain & the Netherlands beat me to. Even better, though, I had two secondary lines of colonies & trade posts, one north & one south, that TOTALLY BLOCKED OFF the interior of America east of the Rockies. In other words, by 1608 I had guaranteed that ALL of America except for the 4 Spanish/Dutch provinces would be English. I have played up to 1669 now & my blocking lines have remained entirely effective. I have been expanding inward (i.e., toward Ohio, Missouri, Mississippi, etc.) at my leisure, balancing this expansion against my other colonial needs (India/Indochina & California/Hawaii).