What if Germany rained rockets into London? Did it. Done that. What did Great Britain do?
Before I answer that, here is a fact: During 1940-1945 some 60,000 British civilians were killed by German Nazi aerial bombings (43,000 from September 1940 to May 1941).
What did Great Britain do? The British RAF dropped over 10,000 tons of bombs on Brunswick and Duisburg. 2,500 civilians were killed in Duisburg alone. That is just two cities. I did not have time to research more of the British retaliation. However, history is a strict teacher, which is something the typical college grad has no clue.
Do you know about the British bombing of Dresden, Germany?