The Sun and Regulus together at the summer solstice Viewed from Stonehenge, when did the Sun and Regulus rise together at the summer solstice? Also when did Regulus rise before the Sun and would have been seen before the dawn glare?
Using sky simulator software (in my case Guide 9.1), the sun and regulus rose at the same time in about 2500 bc.  I think you would need to go back another 1500 years to be able to see regulus in the pre dawn glow. At that time, the sun would be about 7 degrees below the horizon as regulus was rising.

However the surrounding woodland wasn't cleared until about 5500 years ago (per Wikipedia), so it may not have been possible to make this earlier observation.