Taxis are a good option for groups of people sharing the fare, which can be steep - the meter starts ticking at £1.40, and rises by 20p with every 219 metres travelled. You can hail a taxi by raising your arm when you see the yellow sign above the windscreen illuminated.
Be aware that evening rates are more expensive, with a 60p surcharge kicking in at 8pm, up to 90p after midnight. Tipping at about 10% is expected.
Note that it can be difficult to hail a cab in popular tourist or nightclub areas, and drivers are perfectly within their rights to refuse a fare, especially if they suspect you've been partying hard. You can phone Radio Taxis at 7272 0272 to order a cab, but you'll pay an extra £1.20.
An alternative to the traditional black London taxis are minicabs. Despite the name, minicabs may offer space for more passengers than a black taxi; traditional London taxis can seat 5 passengers, while minicabs seat from 4 (most common) to 7 people. These cabs are unmarked, and cannot be hailed on the street like a black taxi; minicabs must be called for by telephone from a Licensed Private Hire Operator.
Minicabs do not have metres, they charge by the mile and at the current time prices vary between £1 - £1.50 per mile. Ask for the fare when calling for a minicab, and verify the fare with the driver before starting your journey.