Pricess cruises do a round Britain trip as do Page & Moy with Ocean Majesty and their's includes sailing up the Thames and under Tower Bridge (open). My in-laws did the Princess trip this year and enjoyed it. Personally I'd avoid Marco Polo until they get their act together.
Went on maiden voyage of Marco Pole last Feb time to Fjords . They had just taken on a european crew who didnt have a clue on service etc. The restaurants didnt have enough seats particularly at lunch time and menu very limited.
Improved as time went on but crew very unfriendly. Did enjoy cruise as destinations good and did enjoy ourselves anyway.
This ship entered service in 1966, 43 years ago so how could you have done the maiden last Feb?....Wilba.