Ok - its 5 days - so not a massive trip - also it's the UK, so it's easy to transition into as it's 1st world AND English speaking (mostly...lol).
Now - firstly - condolences on being a Man U supporter - secondly, I'm assuming you'll be going to Old Trafford?
You'll most likely be flying in to London, so you'll have to work out how to get to Manchester from there - you can either drive the M1 (which remember the steering wheel is now on the CORRECT side of the car), get a coach, or fly.
If you book early enough, Ryan Air or Easy Jet can be REALLY cheap - as in a pound cheap.
Money wise - don't worry about travellers checks etc - just use your credit card and your debt card - you'll get bank exchange rates which are the best you can typically get, won't get ripped off by money changers etc. Just get off the plane, find the nearest ATM in the airport, and get yourself some local currency if you need it.
Absolute worst case just get $100 in local currency at your bank if you really feel you need something before you fly.
It's only a week, so no real stress here. A good reference is
http://www.lonelyplanet.com/, the books and forums are great.
I've done a lot of traveling so feel free to ask mate.