There are lots of spots to find views of the Palace of Westminster.
If you head down onto the waterfront area either side of Westminster Bridge (on the opposite side to the Palace), there is a little underpass under the bridge. From here you get great framed views of the building. The lights on the bridge are a great focal point as are any passing boats. This is the perfect spot for sunset into twilight as the street lights start to come on. Long exposures are fantastic to give the River Thames a silky surreal feel.