Park Plaza Westminster. I only ever stayed here once b/c I'm a Marriott loyalist and Park Plaza is a Radisson property... but my company held some meetings there 3 yrs ago, so they booked rooms for everyone who was coming in from out of town.
I don't recall the rooms being particularly anything unique or memorable. I suspect most people coming into town wanting an experience would book a better room as this hotel does offer great views of the London Eye if you're willing to pay for it. But like I said, my company booked these rooms so we got the lowest budget rooms (I'm sure) that were on a lower level with views of the hotel immediately across the street. I remember co-workers complaining about how cheap the company was not to spring for slightly pricier rooms with much better views... b/c who wants a room that faces another hotel?
But if thats what you're really looking for, this hotel would fit the bill for a night. The windows are floor to ceiling. The bed in our room wasn't particularly close to the window, so you really do have to stand in front of the window with the lights on if you want to put on a show. And you are pretty likely to attract an audience. My gf and I had fun! With that said, this truly was the only memorable thing about this hotel.
Honorable mentions:
Wellesley Knightsbridge. This is one of my favorite hotels in London. There might be rooms that face other hotels... I don't know b/c I've only ever stayed in the same room that faces Hyde Park. With that said, if you stay in a lower level and want to put on a show for anyone walking down the street who might look up, you can. I haven't done that here, b/c my favorite thing about this hotel's rooms are the bathrooms. They're setup for porn-quality sexcapades. The room is mirrors and the giant shower is all glass. If you're someone who gets off watching yourself fuck from every angle and enjoys shower sex, you'll love staying here.
For the exhibitionist, the public mens restroom for the cigar bar in the lobby has an urinal with no divider that is positioned in front of the door. When guys walk in, they can't help but to get a full eye shot of your dick.
After I had a few drinks in the bar, got a full bladder and was feeling a little horny, I'd pop in there. I'd rest my hand closest to the door in my back pocket so as not to obstruct... maybe even stroke it a little with my other hand some to get a nice swollen semi... and start pissing away when I'd heard someone start to walk in. Almost every time, I'd catch a good reaction from a guy. I mean.. it's just what you expect to see first thing when you walk through a door, even if it is the restroom! I even once had an employee who walked in stand at the urinal next to me (instead of 2 down, leaving room between us, as is etiquette)... and blatantly watched me. I think he thought I was cruising (I wasn't), but I gave it a good shake for him before making eye contact and giving him an acknowledging nod.
W Hotel London. This is in the Piccadilly area (if I remember correctly). There is an interesting design structure that surrounds the building, so while you can see out the windows of your rooms, they're not the best rooms for window play. I also think most of the surrounding buildings are flats or offices where people don't pay as much mind to what is happening out their windows as tourists do.
With that said, I mention W London for two reasons: (a) thin walls and (b) great spa.
The walls are very thin. It's a real roll of the dice if you want a place to lay your head for a guaranteed goodnight rest. But, if you're someone who gets off on loud sex and being overheard, you might get just as lucky as we did and find yourself competing with the strangers next door for loudest, wildest night.
The spa also has a great, tiny little sauna. It's shaped like an L, so you can sit out of view of the door. I like this b/c I've been in hotel spas where sitting in view of the door was the only option and watched guys walk up as if they were going to come in, but then pause and turn when they noticed a big sweaty cock staring them down on the other side of the door. When you sit out of view, even the most timid men who would prefer not to share a small hot space with another naked guy aren't prone to turn around and leave once they walk in and realize they're not alone. They've committed.
I enjoyed sitting on the top level, in the corner of the L. For one reason, b/c I could really manspread and prop my cock/balls on the edge of the bench so they're well displayed to anyone who walks in. Second, it made it so I had to actually stand up and move to one side or the other so there was adequate room for others to sit.... practically swinging my dick in their face as we maneuvered around each other in this cramped, damp space. At that point, the ice is broken and it's easy to get someone into conversation.