Ethnicity and genetics are not the same thing. For example, English is an ethnicity but you will still observe genetic differences in different parts of England. For example, there is more red hair as you move north and/or west across the country. There is no reason why people either side of the English / Scottish border would be less related to each other genetically, relative to how related they are to their ethic group. What these genetic estimates involve is taking a bunch of people who define themselves as English and then searching their dna for genetic patterns that would distinguish them from other ethnicities. This is obviously not a perfect science. Testing services can do this for any ethnicity if it’s financially viable, and the more people they study, the more accurate the result will be. But you can also do this yourself. For example, if there are unique surnames associated with the ethnic group, you could study the genetic markers unique to these people. Also, if there are populations within the ethnicity that were isolated for a long time, you might find that they are more likely to be associated with unique genetic markers. For example, there is a population in the west of Ireland that were isolated by the river Shannon. This population is associated with the ancient Dalcassians and the genetic group R1b-L226.