I’d definitely do it, it’s 10 hours daytime flight so you can maximise the value. We had a brilliant flight in F on LHR-SEA last summer, though on a 777 rather than 747. We were on the early flight - BA53 - that left London around 0930. CCR for breakfast, buddy dining for lunch over Greenland, snooze over Canada then early afternoon arrival into SEA. One of the best BA flights I’ve had. I’d grab the seats now. Shouldn’t be any difference in taxes and charges for this.

no idea about refund of seat fee I’m afraid.