We are on September 5 sailing so looks like we I’ll go before you. We are going to also do 2 days on each end. We did a British Isles cruise a few years ago and spent five days in London. We stayed at Marriott St Pancreas and used hop on hop off bus. We got to see London and it was super convenient. Tube system is great and you might want to det an oyster card similar to Marta card for using subway.