A wholesaler who makes deals with the airlines on flights is called a consolidator. These tickets can often be cheaper then buying direct from a particular airline and are sole online and by travel agents. These tickets can be a amazing deal but can also come with restrictions. These tickets are non-endorsable but this rarely happens. This means that if your airline cannot get you home, no other airline is obligated to honour this ticket.
Frequent flier miles are usually not allow on these tickets either. If your carries lowers their ticket cost, you will still have paid for what you have, and now that might be higher than what other travellers are paying. If you are searching online for flights to London and find a consolidator fare, be sure to ask about the cancellation policy and what restrictions are attached to the ticket.
Budget flight also have their restrictions as many are non-refundable and unchangeable. Changes can be made but a penalty of usually two hundred dollars is charged. If for some reason you need to change something about your flight, just call the European office of your airline. If getting home early is needed, go straight to the airport and speak with someone. In person works better than on the phone.