Welcome! Sounds like an exciting project. 
A few thoughts:
- Switches
Which switches did you look at? The “friends of hue” group has some quite nice ones which look like regular U.K. switches:
- Reliability
You’ll need to define what “fairly reliable” means to you. The SmartThings platform generally needs quite a bit of tinkering; there are a lot of undocumented changes which impact customers. There has historically been a minor outage every month except one or two for the last two years. You can neither deny nor defer changes. We usually get a few days’ notice of maintenance outages, but not always. 
every month there are new threads from people saying there setup had been working fine for months, they themselves haven’t changed anything, but now it’s stopped working. 
Most glitches don’t affect everyone, but some do, and it’s definitely not a “set and forget” system.
My own personal requirement is for an MFOP (maintenance free operating period) of at least 6 months and preferably a year. The Hue bridge does that easily, as does Amazon Echo and Apple’s HomeKit. SmartThings has yet to come close.
The following is a typical example of a glitch from this week. This person’s system worked just fine for years. Now it doesn’t.
And another:
Fibaro Single Switch 2 no longer responds to status (January 2020)
- Security Systems
Because of this unreliability as well as the cloud based nature of SmartThings it’s simply not fit for purpose as a security system. (For one thing there is no way, none, to arm/disarm the system if the internet is out.) That’s not just my opinion: the company says so themselves in their product usage guidelines:
https://www.smartthings.com/guidelines
Data accuracy and consistency from SmartThings sensors, including those provided by SmartThings directly, resold by SmartThings, or supported by SmartThings, is not guaranteed. Therefore, you should not rely on that data for any use that impacts health, safety, security, property or financial interests.
So if you are looking for a security system, keep looking. Yale Home is quite popular in the U.K.
And the Ring Security System has recently become available for the
UK. It gets excellent reviews in the US, I haven’t yet had a chance to look at many reviews for the U.K., but most seem similar to the US.
- Fibaro makes excellent UK smoke sensors. Although if you go with Yale or Ring you may want something else.
It’s also important to note that SmartThings does not have cellular backup, so all notifications, whether to you or a monitoring company, require the internet. That’s not true of Ring or Yale. So most people add fire monitoring to their separate non-SmartThings security system. And again there’s the whole reliability issue.
4. In wall Sockets for the UK
It’s just a very small market because of the different plug styles and some differences in wiring and safety codes even among EU countries. So most manufacturers have opted for the plug in “pocketsocket” style devices which only require a simple case change. There are a few exceptions, most popularly LightwaveRF:
https://lightwaverf.com/pages/smart-power
These do have a cloud to cloud integration with SmartThings as long as you get the 2nd generation or newer. (They also work locally with Apple HomeKit)
Energie MiHome is a French company with a U.K. range that includes inwall sockets. Like LightwaveRF, they use their own proprietary protocol so the integration is cloud to cloud. I don’t know if they’ve released one for SmartThings yet or not. Most people in the U.K. seem to prefer LightwaveRF.
(Note that there is also a Chinese company with a range called MiHome that is entirely different.)
But other than those two, I don’t know of any well-engineered devices intended for DIY installation. There are some cheap Chinese WiFi models without safety certifications, but I’d be concerned about fire safety.
Another option for the EU which is not available for the US is an inwall micro that goes behind the existing dumb socket. In the US, the power draw for outlets is a minimum 15A, too high for most micros. But in most EU countries it’s 10A, and the micros are a possibility. Both Fibaro and Aeotec have suitable models that work with SmartThings. Only the on/off binary switches should be used for this purpose, not the dimmers.
- Hub deployment
You can certainly wall or ceiling mount most of these. Check with the manufacturer to be sure.
Every wall or barrier reduces signal strength, so ideally you want them in open air central to the home. Not in a closet or cabinet. And again ideally the hubs should be at least 3m apart to reduce interference. That doesn’t mean they won’t work if they’re close together, but your networks will run most efficiently (with fewer lost messages) if they aren’t.
Start with post 11 in the following FAQ, then go up to the top and read the whole thread:
