Is there any reason why you couldn't do this non-visually? That is, using PhantomJS to determine that a DOM element is within certain bounds for a position using if statements?
Simplicity. This automatically tests the whole page "for free".
Robustness of feedback for time spent. A small visual change could cascade into lots of tests cases that must be updated (rendering is hard, look how long it took IE to get it right). You don't need to update any tests with this tool, you just give the OK to a visual diff with expected changes.
Dynamic content. The BBC homepage is going to be different every day. Writing tests to test this could be arduous and error prone. Testing production content on build 779 vs 780 is a simple way to test regressions.
Also, the difference between a wanted visual change and a break is dependent on the change you want. That makes this quite a flexible tool (it's not just "any difference is wrong") that can be used simply and is unlikely to have false negatives (while making it easy to ignore false positives).
Also, this means it can be verified by a non-technical person. All you need to do is verify that the differences are either 1) wanted or 2) irrelevant, and this is an extremely simple and robust way of achieving that.
They even mention anti-aliasing rendering issues, and applying of a 20% fuzz factor to work around it...