Welcome to the world of retrofit!
If you’re doing some major works on your home I’d get in contact with the Carbon Coop retrofit team, there are SO many options out there, having a (non-sales-ie) helping hand to guide you through it is priceless, and can help you look at your budget and timeline, and then schedule what will have the most impact now and what to do in the future. If you’re looking at underfloor heating and you currently have floating timber floors, it’s definitely worth considering taking that out and putting a solid, highly insulated floor in its place.
A smart meter is a great first step, it’s free (from your electricity supplier), I’m a bit surprised you’ve not got one already if you’re on an Octopus Agile? You can then sync this up to Carbon Coop’s Impact Tracker and get more detail out of your current system than you get from just looking at the bills.
As with most things, monitoring is a bit of a rabbit hole and you can get more detail out of your home, it just depends how much time/money you want to spend to get there, and how techie you are. There are some devices you can clamp onto your main electricity feed and get a second-by-second trace of how much electricity you’re using. emonPi Energy Monitor - Shop | OpenEnergyMonitor is a good example and the Open Energy Monitor team work with Carbon Coop on certain projects.
Best of luck on your journey!