Here's how this particular deal works (via Neowin). If you go to the Microsoft Store UK site and put one of those Surface products in your cart, an offer will pop up to throw in the Lumia 950 and one year of Office 365 Personal for the price of £359.99. That's £40 off the price of the phone itself, which normally sells for £399.99, plus they throw in the free year of Office 365 Personal which is usually £59.99.
Keep in mind that the Lumia 950 already comes with an offer for a free year of Office 365 Personal, which means this bundle gives buyers in the UK two years of Microsoft's productivity service. Also, if you buy the Surface 3, you gain yet another year of Office 365 as it also comes with its own free offer.