the 3870x2 will not be £250, not even close to be honest. its not simply 2 cars put together, its a dual core gpu, 2 cores on one pcb, not the 7950gx2 card.
you will pay a lot for the process of manufacture for the card, as well as the fact as a single 3870 retails for about £155.
also, it will be their new top end, if it competes well against nvidias top end, then they can price it as they used to price their previous top end cards like the x1950xtx.
like you said. 300+, would be lucky if it was on the lower end of £300 as well.
again, people paid and still do pay this for nvidias top end? its enthusiast end, they can charge what they want, people will always buy it.