Overnight in London - Time to see a few sites?

The UK is not part of Schengen, so you will go through immigration on arrival at LHR. It can take a while to get through immigration, although my experience with late evening flights is that the queue has not been too bad. However, you probably won't be at your hotel until 10pm at the earliest. I would suggest that you stay somewhere along the Piccadilly line. Check tfl.gov.uk to make sure that there are no planned works that affect that line during your stay. I often stay in the Earls Court or Gloucester Road area when I have a short stay and am flying out of Heathrow. In particular, I like the base2stay hotel. Earls Court is a about a 45 minute tube ride from LHR. It is about a 15 minute tube ride from Leicester Square or Westminster. For a transatlantic flight, you really need to aim to get to Heathrow about 3 hours before departure. I will sometime cut that a little shorter (2.5 hours) when I've checked in online, but Heathrow can be unpredictable so I would not try to cut it too close. If you were to stay near Earls Court, I'd suggest that you be on the tube back to Heathrow no later than 11am. Since you don't have much time, I'd suggest that you just do things outdoors. If you stay at Earls Court, you could get up early and take the District Line to Embankment. At Embankment, take the exit towards the Thames and cross the pedestrian bridge to the other side of the Thames. Looking in one direction, you will see the Houses of Parliament. In the other direction, you will see the dome of St Pauls. Walking along the path on the river is a very nice walk. Alternatively, you might look at one of the walks in the Rick Steves London book and follow that.

Easy reply. Many of THE sites are within walking distance of one another. Where ever you decide to stay arise early, have some coffee and breakfast then head to the closest Underground station. Trains will be crowded with commuters but don't fret. Your destination is the Westminster Station which on the large colorful Underground route map will show as being on the Victoria (light blue), District (green) or Central (yellow) lines. Pretty easy to figure out but depending on where your are staying might necessitate a train change or two. http://www.tfl.gov.uk/assets/downloads/large-print-tube-map.pdf. Once you arrive at the starkly modern Westminster Station, disembark, Mind The Gap and follow the signage to EXIT 1. There are numerous exits but use this one. Signage reads Westminster Bridge, London Eye, and something else. Climb a few stairs and before you is the Thames with the Eye across the river. Turn right, look up, smile. Climb the stairs and turn right. Once you've seen Parliament, heard Big Ben chime, walked past Westminster Abbey you can then walk along Birdcage Walk past St. James Park up to Buckingham Palace. From there walk thru Green Park and over to Piccadilly Circus or walk back down the Mall to see the Horse Guards, walk through the grounds out to Whitehall and follow to Trafalgar Square. The National Gallery, St. Martin of the Fields are at Trafalgar Square. From Trafalgar easy stroll to Covent Garden along the Strand. Turn left on Southampton Street to get to the covered Covent Garden Market place. From their you could walk to the British Museum or head elsewhere. Will all depend on what you want to see and do in your short time period. Lastly, use Google maps satellite view. Google Big Ben, London, United Kingdom. You get a bird's eye and by using the zoom can see what I mean about sites being close. Enjoy.

Definitely allow the required 3 hours for checkin at Heathrow- the time consuming part is getting through security- the lines can be L-O-N-G especially during the middle of the day. With the limited time you will have available (maybe an hour very late at night, and then a few hours in the morning) i wouldd stay right in the middle of the sights. Piccadilly Circus, St James, Mayfair or westminster will allow you to walk to major sights as trafalgar square, Piccadilly circus, Westminster Abbey, Buckingham palace, Horse guards. (however hotels in all these areas tend to be expensive) Earls Court , although on the piccadilly line, is not within walking distance of any of the sights.