You can book a taxi via following methods:
Online: http://www.relyoncars.co.uk/
Telephone: +44 173 286 3800
Yes, it's possible to pre-book a taxi with Relyon Cars, please contact the company.
Relyon Cars 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.