Get back home to London is what I would consider normal to hear (I live in the west coast in the states. But I don't think either would be considered incorrect.

I am not a native English speaker. But I know the second is wrong. In 'get home', 'go home', 'go back home', home is often used as 'an adverb'. So you don't need a prepositon here. But if you want to say the flag/apartment/house that you live, then you can say 'get back to MY home in London' as Peachey said. In this phrase, 'home' is 'a noun'.

"Get back to home" is not correct. However, you can say, "get back to MY home in London". You can check the difference between house and home with a simple online search. "Get back home to London" is correct.

For me, "get back home to London" is correct. We don't use a preposition between a verb and "home" e.g. go home, get home, arrive home, leave home. Think of "verb + home" as an idiomatic usage.