I've flown BA from ATL to LGW in December for $373 inc taxes - but that was December 2004. I'd say £300 is a pretty good price for that route on BA; Delta can be a bit cheaper at times but then you're, well, flying Delta!
The $373 fare is amazing! I'd kick myself if we spent $600 now and could've had it for $373 after all by December. Not to mention Dec. will be cooler (we're heading to Alabama) and that my knee problems are slowly improving and we can manage to get around, but I can't walk from the the entrance of the train station to the train right now, and their assisted travel at FGW is not too hot in my experience -- maybe by Christmas I could walk that far, I just don't know because my recovery is soooo slow and won't be a full recovery anyway, that's just not out there for me until such a time as docs decide to do knee replacements. We have the added wrinkle that my husband's mom just told us she'll be off work at the time we were thinking of going to Alabama this September, and she could come from N.M. to join us there -- we've never met each other's parents, and this would let my mom and his mom meet, too. We were going to either fly to N.M. as well, or just do Bamam and go to N.M. next year. But his mom's offer lets us both see our moms without as much flying and plane changing, which is a pain in the ars* because I can't walk through the airport and have to wait for an employee to show up with a wheelchair at the gate and it gets stressful as you're hoping you don't miss a connecting flight.
Right now all the fares for December, going mid-week just before or just after Christmas, are pushing $900. I wish I knew when they would change! It seems like I once found a site that showed what fares various airlines charged at different times in the past, but I can't find it now, so maybe it's just wishful thinking that such a site exists!