You can book a taxi via following methods:
Online: https://www.airportstransfer.com
Telephone: +44 207 434 3666
Email: support@airportstransfer.com
Yes, it's possible to pre-book a taxi with London Airport Transfer, please contact the company.
London Airport Transfer 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.