You can book a taxi via following methods:
Online: http://www.onlineexpresscars.co.uk/
Telephone: +44 203 475 5566, +44 208 133 8776
Telephone: +44 780 516 9898
Yes, it's possible to pre-book a taxi with Online Express Cars, please contact the company.
Online Express Cars 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.