Meet-and-greet at your Heathrow terminal, then a private, customisable loop past Westminster, Big Ben, Buckingham Palace, St Paul's, the Tower of London and Tower Bridge, returned to Heathrow before your onward flight.
Nearly 1,000 years of history, the Crown Jewels and a free Yeoman Warder (Beefeater) tour in one ticket. Two to three hours, reachable from Heathrow on the Elizabeth line. No large suitcases.
Hop between Central and East London piers (Westminster, London Bridge, Tower, Greenwich) on the fast Thames commuter boat. A flexible, low-cost way to sightsee from the water on a day layover.
The notes above are specific outings. These are the timing rules that decide whether any of them fit the gap on the ticket.
Two to three hours
Tower of London
The Crown Jewels and a Yeoman Warder tour, if that is what the ticket includes. Allow the ride from Heathrow and a last hop. No large cases inside.
This is the 24 or 48 hour booking. It is not a warm-up for Westminster Abbey.
The short booking
London Eye
About 30 minutes in the capsule, 135 metres up, with Westminster below. A fast-track slot is what protects a tight layover. The standard queue does not.
Do it instead of the Tower when the gap is shorter than a castle visit.
Flexible on a day layover
Uber Boat
A return river ticket between central and east piers: Westminster, London Bridge, Tower, Greenwich. You board when you are there. One to three hours if you let it be that.
Greenwich is the long version. One hop and back is the layover version.
About six hours
Private loop from Heathrow
A meet at the terminal, then Westminster, Big Ben, Buckingham Palace, St Paul's, the Tower, and Tower Bridge, from the car, and back to Heathrow. From about $404.
It fits a long connection when you do not want the railway. It does not fit in front of a separate timed entry.