The biggest problem with this best price guarantee is BOOKING CLASS. Very little number of OTAs have this "letter" showing when you do the booking, and that's the catch. I remember I was doing this best price guarantee claim about a half year ago - found good price with OTA, made screenshots, then bought tickets with Qatar. Then sent it all to Qatar email, and a few days later got refused, saying that BOOKING CLASS letter is missing. Took me a good amount of time and efforts, but in the end I've managed to get a refund - not much, something about £120 for 2 tickets in Y.