Immigration at man airport uk?

Yes hi i was wondering about immigration when i arrive at man airport in the uk for the first time. what will it be like. i am visiting a friend for 2 weeks in the uk she has written me a letter stating that i am allowed to stay with her for my 2 week holiday. and will provide me with food. i also have a round trip ticket back. i will have 200 pounds in cash and more in my debit card. i am from usa. do u see any problems with any of this? any help would be much appreciated. first time flyer. very nervous thanks again!