Panic wastes the one resource you're short of. The question has a real answer, computed the way airlines compute it: count backwards from bag-drop close, not from departure. Here's the arithmetic, airport by airport — and the fastest next move whichever way it lands.
Send the flight number — we compute the deadline for you
Bag-drop close: typically 60 min before long-haul, 45 min short-haul, and 40 strict minutes on Ryanair and Wizz. Hand luggage + checked-in-online moves the deadline to gate close, ~30 min before departure. This number, not take-off, is the finish line.
Not the best case — this hour's case. Off-peak: central London to Heathrow 35–50 min, Gatwick 55–75, Stansted 55–70, Luton 45–60, LCY 20–35. Rush hour can double the first three. Add 10 minutes for kerb-to-desk inside the terminal.
Sum beats deadline: leave now, book while moving, finish packing in the hallway. Sum misses it: call the airline before the flight closes — flagged-late and same-day rebooking are both far cheaper than a silent no-show.
Standing beside Victoria, Paddington or Liverpool Street with an express leaving inside five minutes? Take it — a train doing 30 non-stop minutes beats anything on wheels. Anywhere else, the maths flips: getting to the express usually costs more time than it saves.
WhatsApp us the flight number and pickup postcode. You get back a fixed quote, a realistic pickup time and — this matters — an honest read on whether the deadline is makeable. If it isn't, we'll say so before taking your money. Meanwhile the empty-handed sprint to a station you're not sure about burns the minutes that decide it.
Check in online if you haven't; screenshot the boarding pass; move bags to hand luggage if the fare class allows dropping the checked bag entirely. Airside, know your gate direction before security spits you out. Ten minutes of admin in the back seat buys back ten at the terminal.
Call the airline before departure — same-day rebooking and rescue fees exist and beat fresh tickets badly. Then book the next flight's transfer calmly, with the deadline arithmetic done right this time. Our when-to-leave guide makes it unrepeatable.
Honest answer, fixed fare, driver moving while you zip the bag.