I suppose the buck will be more efficient than the linear regulator.
But also, assuming they are serious/honest/realistic about the 50,000 hour (burn) life. It helps keep the heat (hence lowers component temperature) dissipation down, possibly widens the range of (mains) input voltages the bulb(s) can support, and could reduce warranty claims (early failure).
The lamps are potentially worth the £12 (as BigClive mentions), because they are, (apparently) flicker free, very minimalistic energy consumption, very long lamp burn life, great for places where it needs to be on most of the time (e.g. some hall ways), and difficult/hazardous lamp locations. Such as high up on buildings.
I hope in the coming future, other suppliers (in addition to John Lewis), will stock those bulbs in the UK. Also, I hope the higher (equivalent) wattage versions, namely 75 watt and 100 watt versions, will also become available (UK). As 60 watts (equivalent) ones, are not necessarily bright enough, for all lighting needs.
I'm sort of stating the obvious here. But the current climate of sky high energy prices, and huge interest in big energy savings/efficiencies, would seem to make these
new ultra efficient LED bulbs, suited for the big time, mass-market. So I hope Philips do market/sell/stock them, much more widely, than they currently seem to be. I.e. More retailers, advertising, bigger range (40W, 60W, 75W, 100W in UK), and possibly reduce the price, as £12, is rather pricey for LED bulbs.