I have a driver on my staff at work, who used to work for one of these companies and if his attitude is anything to go by then trip adviser is right. Grab a pocket a-z such as this one London Mini Street Atlas ISBN-10: 1843486059 / ISBN-13: 978-1843486053 and wander around on your own, everything of any interest is pretty much walkable or a short tube ride away.
We have devised a day for the four Americans(we met on EB Panama repo) that are staying with us prior to the cruise. This consists of London Eye, River cruise, Walking tour to include seeing the following;
London Aquarium
Big Ben & Houses of Parliament
Westminster Abbey
Whitehall
Downing Street
Horse Guards' Parade
The Mall
Buckingham Palace
St.James Palace
Pall Mall
Ritz Hotel
Fortnum & Mason
Royal Academy of Arts
Burlington Arcade
Jermyn St/Bond St
Regent Street
Oxford Circus
Carnaby St
Piccadilly Circus
Statue of Eros
Leicester Square
The National Gallery
Nelson's Column
Trafalgar Square
St Martin-in-the-Fields
Harrods (time permitting)
followed by dinner in Covent Garden and top tickets to Love Never Dies on the Strand
This is achievable in one day......they know it will only be a taster tour, but they only have three days prior to the cruise and we are taking them to Windsor, Blenheim Palace and our local Indian Restaurant (national dish in England is actually now considered to be Chicken Tikka Massala, not Fish & Chips as most people think.
Hope this helps you or anyone else, I will ask at work which bus company is best if you like?
Paul DH of Celestine