...but AFAIK there is no version of Aster 30 in development which is intended to provide the sort of performance found in SM-6.
Well, there is Aster NT (new technology). The Block 1 NT missile is capable of shooting down ballistic missiles with a 1000km range and the Block 2 missile is capable of shooting down ballistic missiles with a 3000km range.
The UK already has a degree of air defence missile capability overlap between Sea Ceptor and Aster 15
That's a good point. I wonder if there has been any thought given to modifying the T45s to carry Sea Ceptor as a replacement for Aster 15. The obvious benefits would be a lighter, cheaper missile capable of being quad packed in a single A50 cell. That would allow for more missiles to be carried with a potentially better mix of long to short ranged weapons. I guess it depends on whether the RN thinks the cost of modifying the T45s is worth the benefits.