I think Diesels are great and full credit to those UK flying mags that keep doing reviews of them!
FADEC is great and decades overdue.
BUT...
A retrofit isn't economical unless the plane is doing some 500+ hours/year, which basically means a flying school, and how many of those can afford to do more than the absolute minimum "VFR legal" maintenance?
Or it means buying a brand new plane, and new piston plane sales in the UK are very low.
Anyone buying one is going to be a guinea-pig, both for the technology and for the UK maintenance arrangements. I would want cast iron guarantees on both fronts.
As an aircraft owner and speaking from some experience on this, I would very carefully verify prior to purchase the exact warranty terms, whether any work can be done locally or whether you need to cart the plane to the dealer for just about everything. A lot of businesses operating in GA are perpetually nearly bankrupt and what you might regard as your friendly local maintenance shop might not be viewed favourably by the dealer, or vice versa. I cannot use my local JAR145 shop for most warranty work because the dealer who supplied the plane won't pay their invoices.
And finally, make sure the plane can be placed on the G- register before purchase. Suprisingly, not everybody checks, one man I know got caught by this. The dealer didn't say anything...
If I was going to start a flying school, I would get a fleet of diesel DA40s, without a question.