In USA heat pump water heaters are quite popular. Without the VAT they can come in under £2k. There are a few advantages such as no need to run main unit all
summer, even can help cooling the property, very little alterations to hot water system needed, sort of plug and play as control system self contained.
Now this I see as a biggie, in cold weather when you need heat the most it doesn't burden the main unit, could be used with economy 7. Or else could be used when daytime temperature is highest. Love to hear comments.