Flight tickets can be really expensive if you're booking them at the last moment while going abroad. Getting a ticket can be tough if you're going to a popular international destination during the "vacation" period or even the "pre-vacation" period. It's always best to book air tickets six month in advance.
There are peak times such as the time before Christmas when air tickets get sold out really fast. Such times are best to avoid as the prices of international flight tickets are generally high. As most people from highly snowy areas travel to warmer or humid destinations just before Christmas is about to arrive, it's best to buy your international flight tickets much in advance so as to get it for cheap.
Airlines allocate air tickets according to what is called as "fare groups". Air tickets range in different prices and according to the fare class you choose. Once all the seats in a fare class are sold out, then they de-list that particular fare class from their price range.
There are peak times to buy air tickets as well. That's exactly when you must buy tickets as the rush in buying air tickets is low. The best time to book international flight tickets is during the non-vacation and non-holiday seasons for any international destination. It is always best to book a ticket in the middle of the week.
If you're going to London for a vacation or leisure, August is the best month to visit London typically because the British people take Holiday during this month. Thus, they leave London during the month of August and therefore the fares become cheaper.
Most airlines sell air tickets in the highest fare group so the prices won't suddenly jump up in price as the flight date arrives. Most airlines have special prices during the off-season so the prices will be altered in order to fill up all the seats in a plane either to compete with their competitors or just drive up the revenue.
It's always best to make sure you create a proper plan of travel and buy flight tickets accordingly if you're going to visit multiple international destinations at one stretch.