You can book a taxi via following methods:
Online: http://www.johnstaxis.com
Telephone: +44 779 290 6559
Email: info@johnstaxis.com
Yes, it's possible to pre-book a taxi with John's Taxi, please contact the company.
John's Taxi 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.