From my understanding of it if you complete your training on the DA42 you get a "type rating" as opposed to a multi engine piston rating that you would get if you did your training on the PA44.
Unfortunately, that is not possible, much as the CAA might wish it. Because the JAA have buried their heads in the sand over the introduction of the DA42, it remains in the MEP Class and that is the rating that must be issued.
All the UK CAA can do is to introduce a temporary requirement, restricted to UK registered aeroplanes, for some kind of mandatory differences training from the DA42 to other MEP aeroplanes. This will have to be temporary as the UK CAA will lose control of licensing policy in a couple of years when EASA takes over and will then be obliged to fall in line with the rest of Europe. Mind you, that might result in even harsher restrictions on the DA42 - I understand that the French DGAC are seeking to restrict Instrument Ratings gained on the DA42 only to aircraft fitted with the Garmin 1000!!