Well ok, I finished my kitchen diner a couple of months ago, coincidentally also the same size and in the UK.
I have two sets of downlights (8 and 6 bulbs), one pendant light, an island pendant, under cabinet lights, a soffit box LED and garden lights, all separately switched, all using Fibaro Dimmer 2s in this room. I have used Schneider Ultimate Screwless momentary switches and grid systems where necessary. Every light switch is retractive/momentary.
I don’t have any HVAC running here at all. I’ve not gone for A/C but I do have underfloor heating. I’ve been waiting to see how things settle in the heating area of ST, and I’m almost positive I’ll be going down the Honeywell route, maybe evohome as I have the option of controlling radiators throughout the house individually (as well as the hot water and underfloor).
The soffit box (https://community.smartthings.com/t/for-fun-pictures-videos-your-setups-hacks/41687/10) does not currently have a physical switch, which I may address in the future. It’s a bit of a gimmick, and not essential lighting, so I may not. It’s done with 12m of RGBW LED strips combined with a Fibaro RGBW controller (soft white shown). I’ve run all three strips directly to the same controller to keep life simple. Some RGBW extension cable used. The picture shown is before I had any paint on the skirting!
Some of my switches have two way setups, e.g. I have one switch for the island pendant on a 4 gang plate by the door, and one switch actually on the island. Not much different to a standard two way switch, but it does give me an option of where to locate the fibaro. This helps with spreading them about. In this situation for instance, I’ve put the module behind the island switch, so that modules for other switches can go behind the 4 gang plate (which currently has two behind it).
In the far corner of the room I have a double switch containing two modules. This is in an external wall with a fairly decent void left behind the plaster board. This means that I have been able to put a very deep box behind it to hold two modules. One does the garden lights, one does 8 downlights.
The under cabinet lights also do not have a physical switch. Here I’ve used a Fibaro relay module (under cab lights are not dimmable). This is connected to the ST motion sensor, placed strategically near the counter space. Between dusk and dawn, if you approach the counter the lights will come on for a couple of minutes.
If I wanted further physical switches, I could do this very easily. I’ve only just started to experiment with some brilliant code developed by @hajar which lets you use S2 on the fibaro module as a smart switch. In my lounge I have a lamp with a smart bulb (Aeotec RGBW Z-Wave) which is on constant power. I removed the single momentary switch for the main light (connected to S1 on the fibaro) and fitted a double, S1 to the main light/first switch and S2 just to the second switch. With the device handler and a smart app, the second switch now sends a signal to ST to toggle the lamp on/off.
In the same vein, I could add another switch in the kitchen diner to do pretty much anything. E.g. setting the soffit box to a particular colour, dimming other lights, turning on the coffee machine and playing a particular tune on the Sonos could all be done on the flick (or double click) of a switch.