What would the use of building a follow on to the 42? The missile and radar system were redundant, and the manning costs would have been much higher - the 45's have a far lower cost of ownership due to automation and their engineering arrangements among other innovations.
Again, the major issue with the 45 build timing was that we spent time and money getting involved in Horizon - it's nothing to do with the 45 being a new design per se.
If you want to rewrite history using 100% perfect hindsight, and needed a more rapid follow on to the type 42, then the easiest way to deliver this would have been to start an independent design immediately, in 1990 or thereabouts, saving five years of delay and about 200 million in design work. That would have delivered us a working AWD sooner, with a slightly more relaxed build cycle and we'd probably have ended up building a few more as at that time the requirement for a number of AWD's seemed more pressing.
There's nothing wrong with new designs, and avoiding them is a false economy - we needed something bigger and more capable than the Type 42, which had already had it's hull extended and displacement increased in the third batch.
The summary of Horizon is worth reading here:
Navy Matters | Type 45 Section
Ian