You can book a taxi via following methods:
Online: http://www.airexpresscars.co.uk/
Telephone: +44 208 571 5151
Yes, it's possible to pre-book a taxi with Air Express Cars (open 24 Hours A Day), please contact the company.
Air Express Cars (open 24 Hours A Day) has both regular and air-conditioned taxis, please mention your preference at the time of booking.
Please contact the company on their helpline number to make any changes.
Generally drivers will meet the passengers at the arrivals on airport pick ups but do check with the company.