Silly people! You cannot convert pounds into US dollars. That's not how it works. They have high tariffs and VAT and a whole bunch of other factors.
The Nexus is suppose to be a budget line. The Nexus S, Galaxy Nexus, and Nexus 7 are all upper range phones with budget prices because they skimp on things like the bestest best camera (or no back camera at all on the Nexus 7) and storage memory, but preserve high performance, great screens, and decent if not remarkable build quality. The leaks of the Nexus 4 and Nexus 10 seem to follow that trend. Since the Galaxy Nexus was priced at $400 and overall well received, I don't see why Google would change the price. They are aiming for the bottom after all, while still maintaining great performance and quality. The Nexus 4 doesn't have any stellar features that would warrant a price increase. Lots of budget-y high-end phones have the S4 Pro so it isn't super pricey. The Galaxy Nexus also had the first 720p SuperAMOLED display, whereas LG has just popped another 720p IPS display on the Nexus 4, in a market flooded with 720p IPS display phones already.