Why do many movie titles differ between the U.S. and the UK?

Why are movies titled differently in the UK?

For example; The Avengers changed to Avengers Assemble.

Similarly Vertige is called High Lane.

What is the reason why so many movies are retitled for the UK?

I know the reason why the title of The Avengers was changed in the UK, but what is the overall reason for changing movie titles for the UK?

Answer

The UK is a different country from the USA, with a different culture, language, licencing laws, censorship restrictions and different existing properties.

Sometimes films in the US conflict with one of those and get a very different release in the UK.

An example is The Boat That Rocked - in the UK it got quite a clever poster that riffs on the Beatles:

In the US it was renamed to Pirate Radio and the poster changed to include the love-interest and a boat full of girls (and is a layout mess):

Yet another example is The Madness of King George III - it was renamed for the US release because while Brits are used to referring to kings by numbers the Americans aren't, so they didn't want to confuse audiences into thinking it was a sequel.

They can make very odd marketing decisions too: the film Couples Retreat got strange racist posters the UK but not in the US..

Fact is that the UK and US are very different markets, so films get different names.