MCV arrived at that number after Sony said that the Slim's launch sales were 20 percent behind those of the PS2 Slim, which sold 50,000 units in its first week in the UK. The estimated percentage of an estimated amount means that the 40,000 number isn't officially official (don't use it as the basis for a chapter in your History of Console Sales book), but it shouldn't be that far off. And that number doesn't include however many PS3 Hefties were purchased.
Compare this "drastic" sales boost to the 7,350 percent bump the Slim caused in Japan. The PS3 went from selling fewer units in Japan than in the UK, to almost four times as many. Now that's a new hardware launch.