Best Airline?
MS
The same airline isn't always going to be the cheapest. Prices will vary by dates of travel and departure city. If cheapest is what's important to you, then use a site like kayak.com, enter your travel dates and departure city, and then see what is the cheapest.
Rona Lachat
Make up your mind. Do you want the BEST service or the CHEAPEST. Do you want the FAST EST flights or the cheapest way possible with a few stops along the way. If you want CHEAP why are you picking the most expensive Airport in London to fly into? Best rate from Seattle can be a different Airline than from Miami . It is expensive from Cheyenne. with no direct flights. Did you want the Airline with cheapest seat? Did you want a seat that includes FREE luggage? Most of the low fare flights go to the other London Airports.
Yumiko
check on Google Flights, Kayak, and Kiwi