You get the best deal from Travel agent. Try this website
http://www.travellink.co.uk calling them is better.
Travel Link
6 Brick Ln
London
020 7375 1661
another good one is
Surma Travels PVT UK LTD
61-63 Brick Lane
London
E1 6QL
Tel: 02072473977
email:
info@surmatravels.com
Both take credit/debit card and will send you Ticket via email. You dont have to go to their office.