I am sorry to hear about your challenges. I myself am quadriparetic, so my smart home equipment is all selected to help me with things that I literally cannot do by myself. It has been of great benefit to me.
That said, I had smartthings for two years and ended up moving all of my critical use cases off of it, including lights, because it just wasn’t reliable enough for situations like mine. They have had an outage at least once a month for the last 20 months or so and in some cases more than one. They also regularly push out firmware updates which will take your hub off-line for anywhere from a few minutes to a few hours. These updates can be neither refused nor delayed, which just didn’t work for me. 
I ended up moving all of my critical use cases, including just the regular lights, to Apple’s HomeKit, which then also gave me a backup for voice control. (I prefer Amazon’s echo, but it’s nice to have Siri as a back up.) in the UK, your option for light switches that work with HomeKit would be the lightwave RF series. In the US, we mostly use Lutron.
I do still use SmartThings for some very complex logic situations that are not critical, like notifying me if the guestroom window is left open, rain is expected, and the guest is not at home. It does that much better than anything else in the low-cost price range. But I just don’t use it for things that I need to have work all the time.
You can read the following thread (this is a clickable link) for more information about reliability and of course decide for yourself.
if you decide to stick with Z wave
If you decide you do want to move ahead with Z wave and smartthings, the first thing you need to know is that Z wave uses different frequencies in different regions, and they are not interchangeable. The frequency of your devices must exactly match the frequency of your hub. So assuming that you got UK Zwave frequency devices already, you must use a UK frequency hub. And the newest V3 hub is not yet available on that frequency.
Also, you should definitely talk to @anon36505037 . He is in the UK and his mother uses a wheelchair. He is now working on his second house with SmartThings. What most people do in the UK is to use the zwave in wall micros which can fit either behind the switch or at a junction box. He has done both houses this way, so he can tell you a lot more about it.
https://community.smartthings.com/t/new-house-build-update-uk/131433/
By the way, there are a number of other project reports from wheelchair users , including one from me, in the forum. Go to the quick browse lists in the community – created wiki, look down towards the bottom of that page for the project report section, and then choose the list on “accessible” projects and you will find them. They might give you some more ideas. 
Anyway, welcome again. Home automation has made a huge difference in my own life, and I don’t have a great deal of money to spend. But there’s a lot of technology available these days that can be of very great benefit.