You can book a cab via following methods:
Online: http://londonliverytaxi.com
Telephone: +19784624442
Email: info@LondonLivery.net
Yes, it's possible to pre-book a cab with London Livery, please contact the company.
London Livery has both regular and air-conditioned cabs, 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.