you're still getting the price of the alienware wrong. with the weakest i7, 1gb gfx card, 8gb ram, 256gb SSD you get £1,448. (apparently you also get a "free flight" with this. we'll just say that matches whatever "app discount" apple gives you for buying the macbook)
anyway, the price difference here is still £80 - and once again, this is with the Apple student discount (assume once you're not a student you're going to think about the real price?). dell is a bit stupid and won't let me copy the settings for the computer in the URL but you'll have to build it up from the cheapest laptop...
so you say that £80 covers the Mac having a nicer screen, looking better, having a better mousepad, and having SLIGHTLY better battery life (see: TechCrunch link below)
BUT ignores that the dell has: 3 USB ports, better speakers, optical drive, HDMI port and an ethernet port. This is hardware, you can say that you don't necessarily need any of them because you live in a house with wireless and download all your software and never use your tv with your laptop, but that's akin to saying battery life doesn't matter because i can always find a power plug somewhere.
http://laptops.techcrunch.com/compare/121-830/Alienware-M14x-vs-Apple-MacBook-Pro-15-Inch-with-Retina-Display-mid-2012 (for the dimensions and battery life - the only significant difference in height, weight and battery life is in height, weight and battery life are only slightly in MBPR's favour)
by HDD favours alienware I meant that if you config with HDD, the price difference becomes even higher than the £80 we have with the super expensive SSD. you might prefer a laptop that can boot up fast, but the fact is that it's an expensive tradeoff once again that favours the HDD based laptops. They're £300 cheaper, you don't need to buy an external drive and you don't need to spend one of your two ports plugged in. the main point I was making here that is that you've found a very specific configuration that makes the difference between the two computers £80 - in most cases it's much higher (and without the student discount it definitely is). Well, whatever your preferences are, my main point it -
with the Alienware, you can choose, with the Macbook Pro Retina, you can't. Either way, your computer will be cheaper, though whether the price difference is £440 or £80 is up to you.