any heat pump which also cools is not eligible

That's seriously stupid - it's the same thing except it can reverse the refrigerant flow to transfer heat energy outside vs inside.

To much backward thinking to plan for needing the ability to cool, especially given your heatwaves over the past few years.

I'm in Australia and I don't think you can buy just a (heater) heat pump. They're pretty cheap too - you can find split system reverse cycle easily for under £500 each.

I am genuinely waiting for the grants to finish (or be updated), as it feels like all the installation companies just hiked their prices to take advantage of the grants. I've got the power run, massive radiators, and thick pipes, all ready to roll.

The big one that will really push HPs, imo, will be if gas starts to rise in price relative to electricity.
At the moment, it's about level in cost to run, even though a HP is 3x+ more efficient.

I just did a check on Heat Geek the other day for my house. It'd cost me £7,500 on top of the grant to convert the place to a heat pump 😭

We had one added and also solar and batteries all in one go. It was around £12k in total I think. Check out Octopus energy Heat pumps if you have a momnet. They are more cost effective and also 0% load for a couple of years. They are doing good things to get more people to switch over. I personally had mine before they started making theirs, but I know a few people that are happy with theirs

any heat pump which also cools is not eligible for the £7,500 grant

This was changed last month, but I think only for air to air systems. Any air to air system which happens to also provide cooling is also included now I believe.