I'd take the home assistant route. Very low investment and the possibilities are endless. 

I cannot comment on whether the Grant heat pump will become popular, but if so, chances are the integration will get better. I just had to switch my car integration to a completely new one as the manufacturer end of lifed the previous API and it took me a few minutes to adjust the dashboards.  

• Neither Homely nor Havenwise currently support the Grant that @nickmorgan has specified (Homely does do Grant, but not that model).
• ChatGPT is suggesting Home Assistant should "co-ordinate" with Homely/Havenwise, but neither of them offer that functionality.
• There does seem to be an Andersen integration for Home Assistant but it's a community-created one and looks to me like it has rather limited functionality and in particular doesn't seem to offer the ability to tell the charger to start charging. Crucially, however, it does at least have the ability - at least apparently - to get the charger's status, meaning the home battery could at least be told to change its minimum charge level to 90% or above so the car can never be significantly charged from the home battery.
• The efficiency gains from Homely/Havenwise vs a well-configured and installed heat pump already set up with weather compensation are far lower than the marketing blurb might suggest. If Havenwise ever do start supporting Grant, even those marginal improvements might be worth the modest fee but Homely is a different story. The only way I've seen so far for Home Assistant to connect to a Grant heat pump is via an integration that uses modbus and Homely's local box of tricks uses the same modbus connection that would be needed.
• The ChatGPT report purely looks at the technically preferred solutions and seems to be recommending Home Assistant as the central point. For me, that's a good solid recommendation but then I work in IT. This would not be a good suggestion if @nickmorgan is reticent about setting up a Home Assistant instance; it's not hard but does require a bit of a learning curve which not everyone is comfortable with.
• If we assume (big if, I know, but we have to start somewhere) that the Grant is set up correctly and is running well under weather compensation, the ChatGPT report ignores the option of simply letting it continue like that, adopting Octopus Go as the tariff and then using the charger app and the Fox app to schedule EV charging and home battery charging during the cheap periods. It's not the optimal solution but it's perhaps 80-90% there for almost no effort.

If @nickmorgan is up for having a go with Home Assistant, that's well worth a try, especially since that then provides a platform for other non renewables related stuff like integrating smart switches, CCTV, home entertainment, robot vacuum cleaners and so on. If not, I'd go with my last bullet point above and keep it really simple.

https://chatgpt.com/s/t_68e67dd61724819184db3b6977cf50bf

Try this and add more questions

https://chatgpt.com/s/t_68e67a78ab308191a858e17da27ebd6d

Make/model of ashp & inverter for starters?

Absolutely. That’s one of the reasons I wouldn’t want to make any suggestions yet.

Make/model of ashp & inverter for starters?

I think at this point all that’s really important is whether you’re open to putting a bit of effort into pulling everything together into a cohesive system, and it sounds as if you are. Not everyone is comfortable with the familiarisation necessary, so I wouldn’t want to assume.

In which case, it’s more about deciding what you want to achieve and that means letting your plans bed in a bit. Better you choose your automation based on requirements than choose an automation system and try to find uses for it.

What rate can batteries be charged/discharged at?