Re: How would you politely tell....?
Just fold your hands, bend a bit like the Japanese and instead of 'Namaste' say 'Aslamalekum'. Tell them this is how we wish people in our country.
They would not know the difference.
(The only thing you have to accept is that if you act differently, dress differently(hijab/burqha) - you will be treated differently) It is better to mould and be a part of the society you live in.