Agree w/ Toni unless you are looking for a super plush flat. Try London School of Economics. I stayed at their Northumberland location (which is one that is not apartments - it's just ensuite rooms). It was clean, quiet, safe and 2 blocks to Trafalgar Square making it a breeze to walk or take multiple forms of public transit to get around to many major sightseeing options. My LSE option also had daily maid service, access to coin-op laundry, a business center, a kitchenette, tv lounge, secure card access to the building plus someone at front desk 24/7 - during the day it is the students, at night it's a security guard - but there's someone there that can give you directions if needed (they aren't concierges, so don't expect them to phone in a restaurant reservation or call you a taxi - the rooms for rent there do have phones so you can handle those things on your own). It definitely saved me plenty of time and money not needing to commute back and forth all the time.
As for what to call the Tube… the locals will figure out in under a minute what it is you are looking for. We call it the "T" here. I have also lived in cities where it was referred to as the "El". I've had plenty of tourists (both foreign and U.S) stop and ask me how to get to the subway or the metro and never once have I directed them to the green boxes on the corner with the crappy little commuter newspapers inside, or the equally crappy sandwich chain that was unfortunately affiliated with the creepy pedophile. And if you type "London Subway Map" or "London Metro Map" into Google, you will get dozens of images of the Tube map popping up on your screen.