I need some help

I need to travel the following routing
19/12 KBP-LHR
30/12 LHR-LED
03/01 LED-KBP
I also have to book KBP-LGW/LHR 28/1
LGW/LHR-KBP 31/1. This is for V-Flyer bash.
Prices quoted range from £660 to $2800/pax
Econ class only, no changes
With all the knowledge out there where can I get the best prices using sites/tools etc.
any help would be appreciated
Iain
19/12 KBP-LHR
30/12 LHR-LED
03/01 LED-KBP
I also have to book KBP-LGW/LHR 28/1
LGW/LHR-KBP 31/1. This is for V-Flyer bash.
Prices quoted range from £660 to $2800/pax
Econ class only, no changes
With all the knowledge out there where can I get the best prices using sites/tools etc.
any help would be appreciated
Iain