First get visa and then visit to the airline booking office of your desire and ask for booking on the dates when you want to travel. They will require your passport for booking. It is possible that your airline might be pack in those days when you are traveling so it will be better to contact an traveling agent of your area for tickets.

 Hi
 
You can buy your tickets as normal once you have obtained your Visa.
 
There isn't any condition on UK Visa about travelling tickets. You can buy
tickets of any airlines and travel anytime you want before expiry date of your Visa.
 
If you need further help, do let me know.