When is this trip?
When you say your hotel is near Heathrow how close are you? Walking distance? Cab ride? Bus?
I’m asking because here’s a plan to consider:
Use the 15 minute Heathrow Express train from Heathrow into Paddington Station. At Paddington use a debit or credit card to get each of you your Oyster card. If you didn’t use an ATM near your accommodation to get pounds from Paddington Station walk over to the Pride of Paddington Pub. There’s an adjacent Barclays Bank ATM. Get about 30£ Each and then walk (about 8 blocks) over to and into Hyde Park via the gate off of Bayswater road at the Italian Gardens. Well signed paths will take you into Kensington Gardens where you can see Kensington Palace.
From there walk over to the Albert Memorial and Royal Albert Hall. Head to the V & A museum to have a pleasant late lunch at their cafe. OR if seeking a pub to have lunch at try the small Queens Arms pub in the Queens Gate mews.
Sated walk to the Knightsbridge Tube Station. Use the pounds you got to put money on your Oyster card ( 10£’s on each should do) .
Then for your first tube ride take the Piccadilly Line
( Dark Blue on the map) 2 stops to South Kensington. Exit the train and then take a District ( Dark Green ) or Circle Line ( Yellow) to Westminster. Don’t be overwhelmed by the tube. Large maps on the wall and signboards over head, as well as attendants to ask. Honestly it’s a breeze when you know where you are going to.
When you arrive at Westminster walk out to Parliament Square. Even though Elizabeth’s Tower with the Big Ben bell is in scaffolding you will enjoy recognizing Parliament and nearby Westminster Abbey. After photo ops walk up Birdcage Walk
( again well signed) into St James Park and onto Buckingham Palace. If the flag is atop the Palace it means the Queen is in residence.
Depending on your stamina you could walk past the Palace and the Victoria Memorial into and through Green Park to Piccadilly or head back the way you came.
Edited: and DONT bother with the Hop On Hop Off bus. With London traffic waste of time and money!