Sadly, I can’t tell you for sure if your model still works with SmartThings as I’m in the US and the app will only show me TV models that are compatible only here not ones in the UK. But considering it is a 2019 model and has worked in the past I’m thinking something got corrupted on the TV. If you have the physical remote, does that work if you hold down the directional buttons? I ask as one of my 2020 TVs had a remote problem where you could only increase or decrease the volume by one click of the remote evenon the app. If you held the volume button to quickly change the volume it wouldn’t work. It would work with an old fashion IR remote though, which i had a Logitech Harmony remote that worked just fine. Like you I tried everything and after reading a factory refresh helped another user, I reset the TV as a last resort and magically it worked again.
Another thing make sure your firmware is upto date, I see that there was an update release a month ago and sometimes the update within the TV’s settings doesn’t get linked to the update and you will have to manualy force it via a USB device. If not updated this could be one thing that could fixe your problem.
The next thing I was able to find a forum post on here saying that something very similar if not exactly the same happened in 2020 here: (SmartThings app TV remote control). It looks like it has never been fixed and due to the fact that your model is still using the old remote it may not be fixed. I can’t say what the developers will and won’t do as us forum members are mostly (about 99.99%) customers like you. Sadly, all I can say is do 1 or more of 3 things, factory reset your TV and see if that fixes it, contact SmartThings developers by going to the menu within the app and clicking “Contact us” and describe your issue (remember though you will more than likely only get a tier 1 support person and it may take a while to get your issue escalated to the right department if at all and they may or may not do anything even if it is escalated. This has been my results for a few issues I’ve had.), then sadly you can just try to live with it.
I’m sorry I can be of much more help. I really do wish I could do more, but all I can do is give advice that most of customers have tried for an issue like this.