You can book a taxi via following methods:
Online: http://www.mainlinetaxis.co.uk/
Telephone: +44 142 355 5555
Yes, it's possible to pre-book a taxi with Mainline Taxis Association, please contact the company.
Mainline Taxis Association 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.