Yes, there would need to be a separation between the hot water and the towel tail circuit, for sure, so I was going to ask Mixergy about how this could be achieved. I know they've been working on a system to heat flats using the iHP so I'm hoping it can be retrofitted to run our towel rails.

Thank you for answering all my questions, I appreciate it! Now that I know it's possible I'm determined to make this happen.

Interesting. That makes sense. I was planning on contacting Mixergy to see if I could get something like this set up. Which HP cylinder do you have, out of interest?

Oh, and in answer to your op I hope they concluded that the A2A mini split was the primary heating and the others secondary?

Nice. And does that mean the towel rails heat up whenever the HP cylinder is heating the water, or do you run them at separate times and switch the route using a valve, or something? Apologies for all the questions, I've been wondering about this for a while!

I've been wondering if I could run our towel rails from our Mixergy iHP cylinder. Was it complicated for you to get that working with your system?

Yes, I suspect you're right.

This is worth a watch too. Installs can be cheaper and less disruptive nowadays. (Note the "AI" he refers to here is good old machine learning, not the generative bs)
youtu.be/ENt9fJBHINQ

The Boiler Upgrades Scheme (BUS) grant is available to all, not just low incomes. £7.5k off the cost of installing a heat pump and removing a gas boiler. It's not been stopped, it's still going strong.