There are factual inaccuracies in this article
and I have submitted some corrections, but just to summarise:
There are four Cray XC40s currently installed at the Met Office headquarters. Two modest sized test and development systems called XCT and XCD (although this last one may be renamed), and two larger productions ones, XCE and XCF which will initially run the bulk of the production forecast and climate research work. The two production systems are either currently or will very shortly be in parallel running tests with the existing IBM P7 775s to prove reliability and that the forecast results are consistent.
The larger ones will have their capacity increased when the IBM P7 775s are removed (freeing some power budget) later this year, with the increased capacity coming on-stream in 2016.
The rest of the resource will not be installed at the Met Office headquarters, but in a data centre that is currently being built on the Exeter Business Park. This will come on-stream sometime in 2017, according to the timetable I've seen. This alternate location is because of power restrictions at the current site.
I'm sure that David Underwood would not have provided incorrect information. Maybe something was lost in the reporting?