Your answer is in your question. The English people are friendly., so don't be afraid of saying something wrong. They will think more of you, not less, for speaking their language.
Also, don't be afraid to ask for help. If you are confused and are near someone who looks friendly, explain to them what you are trying to say, in whatever words you know.
Doubting our own knowledge in a foreign language is something we all do. Don't worry about it.
Finally, you have taken the biggest step toward learning English. You are in England. Be happy!