It is good to take some £ with you. My table mates from the ship, stopped at a small cafe while on an excursion, they tried to pay with US$ and credit card, the cafe only took £ cash. The man had to go to the post office for cash and returned to pay.
I have used International Friends ( see below) from Southampton to our London hotel with stops at Salisbury, Stonehenge, and Windsor. They only go on embarkment/disembarkment days with pickups from certain London hotels, LHR hotels, LHR. A great way to see sights along the way to London and being dropped off at the door of your hotel or if a late flight.
I have used Just Airport, a car service, from/to LHR/London hotel.(see below)
Next year, I am staying at Hilton Garden London Terminal 2 & 3, this hotel is attached to airport. International Friends will be dropping us after our cruise at the hotel.
Depending when you go, Buckingham Palace might be open for the Royal staterooms. (See below)
I suggest getting tickets in advance for Westminster Abbey, London Eye, always a line. Sometimes it is cheaper to purchase tickets in advance plus saves time. (See below)
You could take the Hop On Hop Off (HOHO) to give you a general idea where places are in London. There are 2 HOHO companies with the same 3 routes. (See below)
I would not suggest getting the London Pass, we did not get our money worth. It is better on choosing the place you want to see and just paying for it. Go to their web site to see places to visit.
Royal Collection Trust rct.uk