Another one for the "whatever you do, don't try and change your accent" pile.
I've not forgotten the storm that Beryl Bainbridge caused a few years ago when she called on everyone in the UK with a non-Southern accent to drop it in favour of "speaking properly". She was originally from Liverpool and, once upon a time, sounded exactly like you'd expect from a Scouser. These days, she sounds more like the Queen. If there is one thing I absolutely cannot stand, it's those people who take elocution lessons because they think sounding like the Queen makes them a better person. It doesn't! Everyone who ever meets me will know instantly I'm originally from Essex, one of my former colleagues during my days as a PhD student was from West Bromwich and has a powerful West Midlands accent; the head of Analytical Services was unashamedly from Bristol, and with the hint of ooo-aaarr to match. All three of us have doctorates and could talk at length and with utmost authority on our areas of expertise without ever switching to a No?l Coward accent.
So far I've never met any Americans in the UK who have attempted to modify their accent to fit in and not be seen as American; really, there is no need.
However, there's one aspect of university life to be aware of.
I assume you'll be arriving as a postgraduate, which lessens this problem somewhat. But at some stage, probably during one of those trendy-hippy "One World Week" schemes or similar, you'll have to be prepared to run into that very worst aspect of student life - the unwashed, halitosis-ridden, raving revolutionary - wearing one of those nasty fairtrade-alpaca-woolly hats in a vile swamp colour with strings down the sides, probably banging a tambourine and chanting some kind of militant hard-left but otherwise devoid-of-substance slogans without realising what they're talking about. These nefarious types are known for having an obnoxious "AMERIKKKA IS ALWAYS WRONG ABOUT EVERYTHING!" attitude and, wherever possible, should be avoided at all costs. All it takes is one word and they'll instantly assume you've come up to them to wave a Stars And Stripes in their face, praising the qualities of Dubya and tubthumping about going to war in Iran - even if you're a Democrat-voting pacifist.
One American I knew a while back would always tell the hippy fascists he was Canadian... but I'd only use this as a last resort.