First: work out which system holds your item
This sounds obvious but it is where most people waste a day. An item left in a black cab, an item left in a pre-booked minicab, an item left in a terminal and an item left on a plane all go to four different places. They do not talk to each other.
Pre-booked private hire or airport transfer: contact the operator you booked with, not the airport. The operator can identify the exact driver and vehicle from your booking reference within minutes.
Licensed black cab: handled through Transport for London's lost property service. You will need the date, time and route, and ideally the cab's licence number from your receipt.
Left in the terminal: each airport runs its own lost property operation, usually outsourced, usually with an online claim form and a storage fee.
Left on the aircraft: the airline, not the airport. Items found on board go back to the carrier's own handling agent.
Why a pre-booked transfer is the easiest case by far
If you left something in a car you booked in advance, you are in a much better position than someone who flagged a cab in the street. The operator has your booking record, the driver's identity, the vehicle registration and the exact time window. There is no guesswork about which of London's tens of thousands of vehicles you were in. In many cases the driver has not even finished their shift yet.
Contact the operator immediately with your booking reference, a description of the item and where in the car you think it was. Under the seat, in the door pocket and wedged into the back of the boot are the three places things actually end up. Drivers do check between jobs, but a phone that slid under a seat is easy to miss until someone tells them to look.
Be realistic about return logistics. A driver who is now three hours away on another job cannot always turn around. Reasonable options are collection from the operator's base, a courier at your cost, or a handover on a later booking if you are still in London.
Items left in a black cab
Take a receipt every time. The licence number on it is the fastest route to identifying the vehicle. Without it, the claim relies on the driver handing the item in and the description matching. Drivers are legally obliged to hand in found property, and most do, but the process takes a few days rather than a few hours.
Items left at Heathrow, Gatwick, Stansted, Luton or City
Each airport operates its own service and each charges for storage and postage. Two practical points matter more than the details.
First, report it online rather than phoning. Every airport lost property operation runs on a matching database, and a written description with distinguishing details gets matched far more reliably than a phone conversation. Include colour, brand, contents, any case or cover, and anything unusual.
Second, items are held for a limited period, commonly around three months, and perishables and low-value items go much sooner. If you are already back home overseas, start the claim before you worry about shipping.
Passports, medication and anything with a deadline
Some losses cannot wait for a lost property queue. A lost passport needs to be reported and replaced through your embassy or the passport service regardless of whether it later turns up. Essential medication needs a pharmacy or GP, not a claim form. Bank cards should be frozen in the app immediately, which takes seconds and is reversible if the card reappears.
Treat the lost property claim as the slow parallel track, and deal with the time-critical consequences first.
How to make yourself easy to reunite with your things
A few habits change the odds substantially. Put a lock screen message with an email address on your phone. Use a luggage tag with an email rather than a home address. Drop a tracker in anything you would genuinely hate to lose. And do the ten-second sweep before you close the car door, which is the single most effective intervention available: seat, footwell, door pocket, boot.
Arrivals are riskier than departures. After a long flight, in the dark, in an unfamiliar country, people leave things in cars at a much higher rate than they do heading out. If you are collecting a jet-lagged group, make the sweep somebody's specific job.