I totally agree that England has the same right to self determination as anyone else does and in reality should England leave the union then sheer demographics would mean that the union would be at an end. However you're getting into a whole different ball game in the last sentence if the English are deciding who else should remain in or leave the union. Again the UK's govt's stance has been that the people of Northern Ireland can remain within the union as long as the majority of people there wish to do so. I can't imagine that stance is likely to change either by holding them to the union or expelling them from the union against their wishes.