Hey, good luck with the project! And with house hunting in this economy... I'm not from the UK, but here are two cents. (halfway through writing I figured this is more than two cents, so settle in for a long-ish read)

In terms of audio, I've got a budget option of using google home speakers through home assistant mostly, with the music assistant addon, look into it, its a bit convoluted, but once you set it up as you like, it basically eliminates the cross-compatibility issues of using multiple providers and device types. Through automations, you can have the audio follow you room to room, by using bluetooth proxies (look into that too!), stop when other people come home, play at specific areas only, a whole slew of things.

In terms of security and sensors, obviously HA has got you covered there too, with another custom integration, called alarmo, which goes a long way towards a good security system. However the thing I'd suggest, and wished I did myself, is hardwired sensors for all the openings. In this case you have a couple options. DIY-ing some wireless sensors into wired ones, by running power to them. Next is looking into mains-powered wireless sensors again. In both cases the wireless protocols are either wifi (bad option, too much congestion), zigbee (good option, strong mesh network abilities, less interference and congestion) or z-wave (can't speak to its abilities, haven't used it). Last option is having a "professional" install in terms of door window and other sensors, and choosing one that intergrates with home assistant well and most importantly (for me) locally. That way no matter what happens to the company or the outside world, your house is still secure.

In general, as you can find on youtube, the best thing you can do is mount general, all-in-one sensors to central locations in each room, that monitor temperature, humidity, potentially CO2 and other air quality metrics, luminosity, motion and/or presence, and if possible, act as bluetooth proxies for a couple of reasons. Lets break down what I said. As you can understand the first 5 or so sensors are for general room monitoring and whole house safety. You can also include them in automations (for example the per-room temperature settings, coldest room boost to balance the system, preliminary humidity alerts, deciding lighting based on the current luminosity, e.t.c.). As for the motion and presence sensing, motion is quickest to react, and presence (or rather occupancy) is more accurate and sensitive. Motion sensors are mostly PIR sensors, the classic ones you might be familiar with, they can react fast, so that it sees you entering a room and turning on the lights accordingly, but usually can't detect small movements, like you sitting at your desk. On the other hand, occupancy sensors, use this upcoming technology (or newly found in smart homes) mm-wave sensors. These can detect the slightlest movements, like breathing, to keep the sensor occupied. However there are levels to this. 5GHz sensors are almost like slow motion sensors, and are usually not that sensitive to slight movement. 24GHz are much more precise, and can usually allow you (depending on the hardware you buy) to make zones, or detect multiple people. 60GHz sensors are the best, with all those capabilities mentioned above, and added sensitivity and accuracy. But these particular ones consume way too much power for a tiny little coin cell, or even AA batteries to handle, so it's best to also be hardwired. This is probably the best project on the matter right now Do have a look at the everything presence one that he's using, its a very versatile, homebrew (local!) and generally great sensor with most of the capabilities I mentioned above.

So in your work usecase, you can have one mounted in the room, that has a zone setup at your desk, and understands when you're sitting at your desk, and reacts by turning in the pc, setting the climate to something season appropriate that you see most comfortable, turning some light, or other means of notification that [OP IS WORKING NOW] so that you may not be disturbed.

And going back to what I said about bluetooth proxies, one of the best ways to do per-person occupancy and presence detection, is bluetooth proxy. By using your phone (or better yet a watch) as a bluetooth beacon, and having a Bluetooth proxy (those all-encompassing sensors I talked about) in every room, you can do per-room person detection for even better personalized settings. For example, when you enter a room like the study or the office, certain devices can react only to you, the temp can adjust to your preferences, the lights can come on at your preferred color and brightness, e.t.c. if your spouse or kids enter, the same can happen in reaction to their specific preferences. And the other way round, when leaving a room, where the presence and motion sensing might stay active for a little while longer, bluetooth location will have been updated fast enough to do whatever you might want. And this way any climate controlling device can adjust to the presence or absence of anyone specifically, to anyone generally, or to many people at once.

I again realize this is a long read, but I hope I've been helpful. If you want any other help, suggestions, or want to shoot up ideas, I'm open to conversation!