If a house needs a secondary loop it also needs a grown up heating system
that will include a stored HW of some type (vented, unvented or thermal
store).

Having hot water in less than a second is a luxury rather than a
necessity. If it were easily possible with my new combi I would do it.
Since it seems not to be I shall be content with waiting 30 seconds!

Actually, there is some environmental benefit as well as a convenience one,
due to the reduced wastage of water. I would suggest that a secondary loop
that has a boost button that you can press before have a bath, or before
sitting on the toilet (so it is ready when you finish) means that energy is
actually saved as well as water. However, a secondary loop constantly on
connected to a combi is likely to be somewhat wasteful of energy.

Christian.