Confused between astronautical engineering and physics

Hi, hope everyone is doing good. Im about to complete my alevels. I want to study astronautical engineering and cosmology. So I have heard that i can study one thing at undergrad and then switch to other at masters level 
My question is what should i study at undergraduate? 
I have heard that its easier for a phyiscs major to get into engineering at masters but relatively difficult for an engineering student to get into programmes such as cosmology 
Please someone guide me on this, thankyou