Oculus Rift is available in North America now, as the first units have begun to be shipped to consumers, and (overwhelmingly positive) reviews have begun to hit. However, in the UK, Oculus may not be available at retail for quite some time- according to GAME’s financial reports, the retail launch of Oculus in the country is not till June, which is well after the HTC Vive will have launched there.
GAME is the leading retailer of video game hardware and software in the UK, so they probably know what they are talking about. According to their timeline, the HTC Vive will be the first VR headset to launch in the country in May; Oculus follows in June, while the PlayStation VR brings up the rear in October. The Oculus, Vive, and PlayStation VR will cost £499, £649, and £349 respectively, although the former two need high end PCs to run, while the latter needs a PlayStation Camera.