Hi
I was with UNO for years using various modems and the ZyXEL VMG1312-B10A, as a router and in modem only mode, and never had random PPP drops, but no longer with them so can't say if I would be seeing the same as you now. However this was a thing at PlusNet many years ago when they put some of us on LLU, and turns out it was a configuration issue with their gateways and eventually it was fixed at their end.
Have you got a Thinkbroadband BQM set up, is there lots of packet loss or slow responses that could point to an issue with the line? If that chart is all okay then interference on the line at the xDSL level isn't your issue, and most lines in the UK will likely test positive for some interference, its the nature of the beast.
So if sync remains stable, error rates are in normal range, BQM shows no packet loss or other issues, then the problem is most likely at UNOs side (or kit en-route) under the assumption you've swapped your kit and ruled it out. If it started recently then does that coincide with any configuration changes or firmware updates UNO made to their kit, of course you may not have that level of information or it could be changes made elsewhere on Openreach's kit.
If UNO have no other customers they can see with the same issue in their logs then I would think it must be some kit sat in between somewhere and likely other people are affected with other ISPs, unless you can all get together, as a lone person trying to get it resolved, it could be a long battle. Swapping ISPs may resolve the issue, or it may continue, if it continues you will be starting again with a new ISP to get it resolved. Good luck, let us know how it goes.
Regards
Phil