Seemed a bargain to me, next best that has it in stock is Play.com at £10.00.
This combines both the original game and the Forged Alliance Expansion pack.
Supreme Commander: Supreme Commander is the first ever RTS to deliver truly strategic as well as tactical gaming with an emphasis on scope as well as ease of use. Set in the 37th Century you command one of three races as the Supreme Commander with a single goal in mind - to end the 1000 year Infinite War and become the reigning power supreme. For a thousand years, three opposing forces have waged war for what they believe is true. There can be no room for compromise: Their way is the only way. Dubbed The Infinite War, this devastating conflict has taken its toll on a once peaceful galaxy and has only served to deepen the hatred between the factions. Delivering one of the most advanced and innovative features of any RTS game, Supreme Commander establishes a breathtaking new standard for sheer scope and scale of gameplay. Massive battles unfold across maps of unparalleled size and scope and thousands of units fight to the bitter end.
Forged Alliance: The last days of man are at hand. Two years after the Infinite War the once great warring nations now lie in ruins, and humanity's hope for a brighter future is nothing but a bitter memory. A new, seemingly unstoppable enemy, supported by the zealots of The Order, now seeks to eradicate mankind; UEF, Aeon Loyalist, and Cybran alike. With their backs against the wall and staring into the abyss, the tattered remnants of Humanity's forces must put aside old hatreds and band together as they prepare to make one last desperate stand. One last chance. An alliance forged in blood, steel and hope, they turn to face the dark.