You can book a taxi via following methods:
Online: http://www.barnesminicabs.com/
Telephone: +44 208 741 2044, +44 208 748 6265
Email: info@barnesminicabs.com
Yes, it's possible to pre-book a taxi with Barnes Car Service, please contact the company.
Barnes Car Service 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.