Exactly this. It's a pointless rule if, like on many flights, they simply board Group zero to three in one big lump. BA consistently fail to enforce priority boarding, so whilst this might work during the trial on the selected destinations, I suspect the day to day reality might be less exciting. If you can walk up during pre-boarding then you will need to get there pretty early, and then join the gate bag checkers. I guess we will have to wait and see.