I agree you are probably not on any list of 'people to be given a hard time.' Have your flight itinerary printed out to show you've paid for a round trip ticket. Show them in your passport your record of trips to the US, none of which you overstayed.

And finally, tell them what you'l be paying for your Master degree in the UK. Only an idiot would instead take a graduate degree in the US at many times the price!