There's a lot I wish I'd known to be honest....
First and foremost (and Kristi can attest to this) is to figure out how you're going to pay for the deposit. Without an address you can't get a bank account. So figure out how to get the cash wired from an account in the US in advance. I think you've already got a job, right Sara? Cause otherwise I'd say be sure to have a guarantor available to sign your lease, cause if your name is on the lease but you don't have a job yet, you'll have to have someone guarantee your rent.....
If you can't stand the thought of baths everyday, a shower is key - but not the norm from what I can tell. Most places have hand held showers - but IMHO, that's the same as a bath. LOL. I'm a shower girl, so that was imperative for me. I'm pretty sure a power shower is one that runs on electricity? My df's brother has one and you had to switch on the heater/power unit before you used it. Ours is a handheld shower that mounts on the wall above the bathtub. Not sure what that would be called.
If the flat is on a busy road, double glazed windows are really a must - we live on a very big road and the difference in our front rooms with the windows closed and open is amazing.
We have radiators for heat and I don't know about anything else, really, so can't help you there....
Otherwise I can't think of anything that wasn't a matter of taste, like I really have a phobic hatred of carpeting in bathrooms. And since our house is relatively small, an outdoor space was important on our list. And proximity to the train station was a biggie for us as well since we only have one car.... If I could change anything about our present house (besides the attrocious upstairs colour scheme) it would be the kitchen appliances as our dishwasher is crummy and cheap and I would much prefer a gas hob.
Oh, and be sure to find out what if any utilities are included in the rent - some landlords apparently pay council tax which is amazing! Also, we live in an area that hasn't gotten metered water yet, so that was a big bonus (though we just paid our bi-yearly water bill, and it's hard to imagine it being much higher!!!)
Oh, and I know we've talked about areas before, but I guarantee you that Uxbridge is less than a lot of the rest of London, and the farther out you get, the less it will be. For a small two bedroom with a garden we pay about the same as you probably would for a one bedroom without a garden further in. And though the train isn't cheap, we're only 35 mins out of Marylebone....