Always use official, licensed cabs or trusted car services—and watch that meter
Only use official black taxi cabs or pre-booked minicabs
Gypsy cabs, unbooked minicabs, or otherwise unofficial taxis are usually rip-offs, and always illegal. As in the U.S., most cities require that true cabbies get a license and their cab a "medallion" number and a meter to keep track of your fare.
Ones that don’t have a medallion are usually run by some branch of the local underworld, and they can also charge whatever they think you'll pay, since there's no meter.
You can use "minicab" services, but only if you have booked it ahead of time. Don't just hail one on the street, or accept a ride from one who pulls up to the curb and offer. Yes, usually you will be fine, but sometimes you will be ripped off (and it is always illegal).
Watch that meter
Taxi drivers—even legal ones with meters—will sometimes try to get a fat, unintended tip out of you. If the meter is not on, insist that it be turned on. Make sure it corresponds to whatever per mile/per minute rates are posted (legitimate taxis always have the rates posted where passengers can see them).
The initial "flag-fall" charge plus a per-mile amount (or charge for time stuck in traffic) is standard across the U.K. However, you're taking a long trip—say, to the airport—taxis will often all charge a flat fee, and this is legit.
Check your guidebook, ask at the tourist info office or your hotel desk, or go to their airport's own website to find out what this official airport rate is (for the major cities, I'll provide the going rate on each city's "Getting There" section).
The following small, extra charges (never more than £1 to £6) are often legitimate—but it differs city by city:
- Flat fee charge if you called ahead for the cab (or the flag drops when they get the call, not when you get in the car)
- Charge per bag in the trunk (note: not in London)
- Credit card surcharge
- Charge for travel on a Sunday or major national holiday
- Surcharge for trips to/from the airport
- Charge for trips after hours (usually roughly after 10am or midnight and before 5 or 7am).
If none of those conditions apply, don’t trust any "extra" fee the taxi driver tries to foist off on you.