Kindly observe the following thumb rules before buying your flight tickets:
Do not pay your flight travel agent before you have procured your student visa. There is a cancellation charge to the tune of 3,500 Rs/- and above if you need to cancel your ticket because of a visa refusal or a delay in getting your visa.
If you are confident of clearing your exams and returning in less than 12 months time, it works out economical in taking a one year open return ticket. However, if you are not able to return in the 12 months period, your ticket expires and you tend to lose money on your ticket.
British Airways, Virgin Atlantic, British Midlands, Jet Airways and Air India have direct flights to London from most of the 4 metropolitan cities in India. Although, an indirect flight via gulf countries may be cheaper by a few thousand rupees, it is perhaps advisable to take a direct flight because of the convenience involved. The chances of your luggage being misplaced is lower in the case of a direct flight rather than an indirect flight.
For students who are joining in the September session, most airlines offer student discounted fares and additional baggage allowance. Please check with your travel agent if your particular airlines are offering you the excess baggage allowance or not. British Airways and Virgin Atlantic are known to give over 23 kgs of excess baggage allowance to students.
In general most airlines offer 20 kgs + 7 kgs (of check in and cabin baggage) allowance. Hence, weigh your baggage before you proceed to the airport. If you are carrying excess baggage, keep all the not so essential items in a separate baggage. If the airlines do not allow you to carry the excess baggage, leave it with your relatives / friends who have come to drop you at the airport.
Since, you are travelling on a student's visa and not a seaman's visa, hence you are not entitled to that extra 10 kgs of baggage on showing your CDC.