Westminster Cathedral (Wiki) ⇒ was built in the Byzantine style and completed in 1903. It is free to enter and photography is allowed, although perhaps best to avoid when used for Mass ⇒. The Cathedral is on Victoria Street, very near to London’s Victoria Station.
Westminster Cathedral should not be confused with Westminster Abbey ⇒ which is much older (1089 AD). The Abbey is worth seeing but does charge for entry and does not allow photography.
The lower part of the Cathedral is very ornate but the upper ceiling and walls are bare dark brick. I found it best not to look.
There is a small museum of religious regalia and a lift up the tower to a viewing platform. There is a small charge for each of these extras, paid for at the Cathedral shop.
Inside Westminster Cathedral
Westminster Cathedral Museum
Westminster Cathedral View from the Tower
There are views in every direction from the tower. I thought this one, to the East, was the best. One can see the Palace of Westminster ⇐ in the centre and just beyond is the Shard of Glass. There is a tremendous view from the top of The Shard ⇒ but they do charge (it is worth looking for the online and half-day saver tickets).
Westminster Abbey (the white building with two towers) is just to the left of the tallest crane. The tall brown building is Transport for London which is not presently open to the public (information thanks to comments by Simon J Kyte).