Interesting! I just took a closer look at the technical data for the boiler in question....
The Greenstar CDi Series at a glance - Worcester Homeowner area
Yes it's a 42KW boiler, but the full 42KW is only available to the hot water. The central heating gets from 9.4KW up to 30KW. This is sounding more like it. The 30KW is only 5KW above the radiator capacity.
I couldn't help but notice though, that the 30, 37 and 42CDI models all give 30KW to the central heating. So you pay your extra money just for the better hot water flow.