Okay, a few days in and I'm discovering a few things with my setup. I thought it worth documenting this in case any new or prospective UK users appear, because I'm certainly going to be singing this systems praises.
I ended up having a lot of success with the Xiaomi Aqara QBKG03LM and QBKG04LM wired wall switches (no neutral) and just my original AlertMe SPG100 UK outlets pictured above. They paired perfectly and worked reliably. However, the same couldn't be said for any 'sleepy' battery powered Xiaomi device; basically anything with a model number starting with WXKG. They all dropped their connections after about an hour or so.
On the advice of this thread I ran out and bought a bunch of IKEA Tradfri Repeaters. I reset my hub, moved my 2.4GHz WiFi to Channel 1, set Zigbee to Channel 25 and created a very simple network with one repeater close to the hub, then added a wireless wall switch. This worked and the wall switch stayed connected throughout the day, which had never worked before.
This success continued until I started adding the V1 devices again, at which point the battery powered Xiaomi Aqara devices became hugely intermittent and the V1 end point devices would drop off, sometimes all the way back to pairing mode, after about 10 minutes. All V1 motion sensors and buttons would initially work just fine, reporting motion or presses, but then abruptly stop responding.
This happened repeatedly, so I can only conclude that original AlertMe end point devices do not like sharing a network with Aqara ones. You may have more luck if you don't use any Xiaomi Aqara WXKG or MFKZ devices, but the good quality and comparatively low price means you're probably going to want to.
BTW, I also had no idea what "success" actually looked like for these Xiaomi Aqara devices. Basically, they should work on every button press with maybe a second or two latency (at worst) for the end point to be triggered.
Considering the amount of AlertMe gear I have, I'll be buying another Hub (when I can find one) which will exclusively deal with that kit and control both with the official Hub Link app. Because the UK AlertMe kit was built with security system use in mind I'll refresh the batteries in the SPG100 smart plugs and make a mini UPS for the Hub, so that side will continue to work during a power outage, as originally designed.
Overall I've been very impressed with this system, but even more so with the community here, who've been supportive and insanely quick to respond! Thanks, everyone. 
