Is it important to say “please” and “thank you”?

Yes, is very important for the british people, because they are very polite and educated. “ please”, is a polite manner to ask some thing and when you say "thank you", you are showing your gratefully for some thing that you have gotten. It is considered rude if you don’t do it.
In Colombia, we practice this rule. The colombian people always say “Thank you and please”., is a matter of basic education, respect for the other person. Our parents teach us since our chilhood.