The table shows typical fares and journey times based on distance for three types of tariff. Fares and journey times may be higher if there are delays or heavy traffic. 

Distance
 Approx
journey
time
 Monday to Friday
06:00 - 20:00
(Tariff code 1)

 Monday to Friday
20:00 - 22:00
Saturday and Sunday
06:00 - 22:00
(Tariff code 2)

 Every night
22:00 - 06:00
Public holidays
(Tariff code 3)

 
1 mile
 5 - 12 mins
 £4.20 - £6.20
 £4.60 - £7.40
 £5 - £7.40
 
2 miles
 8 - 15 mins
 £6.60 - £9.40
 £7.20 - £10.80
 £8 - £11
 
4 miles
 15 - 30 mins
 £11 - £15
 £13 - £18
 £15 - £19
 
6 miles
 20 - 40 mins
 £14 - £20
 £18 - £22
 £21 - £24
 
Between Heathrow
and
Central London
 30 - 60 mins
 £40 - £70
 £40 - £70
 £40 - £70
 

There is a minimum fare of £2.20 at all times which includes an environmental charge of 20p to meet the cost of emissions reduction equipment.

How it works:
The meter calculates the maximum fare based on:

Time of day
Distance travelled
Taxi speed
The passenger will be expected to pay the full fare displayed on the meter at the end of the journey unless the driver and passenger agree on the final fare to be charged before the start of the journey.

What you can expect from the driver:
Unless they have a good reason not to, drivers must:

Accept any hiring up to 12 miles or any hiring up to one hour duration, if the destination is in Greater London
Accept any hiring up to 20 miles if starting at Heathrow Airport
Accept any hiring up to one hour duration, if the destination is in Greater London
The driver does not have to accept a hiring if the destination is outside Greater London, if the driver does accept the journey, the fare can be negotiated with the passenger.

http://www.tfl.gov.uk/gettingaround/taxisandminicabs/taxis/1140.aspx