Hey mate, just use some rubber feet with a sticky back. Hobby Craft, Robert Dyas etc. I just use a few to support the MOBO and allow a bit of airflow.
You should be fine if your are using a modern, high quality 3 pin UK plug PSU (Self Grounding) You won’t need to earth the system as the MOBO is grounded via your PSU and the PSU of course has an earth pin(UK). The the 12/5V power cable to the MOBO has a ground pin.
Many modern PSU have all sorts of safe guards build in above and beyond; thermal fuse cutout, over/under-load, short circuit etc etc
If anyone else wants to comment on this one please be my guest?
I’m running 5 open airs rigs with Fractal design PSU’s and no issues so far, i have also been building PC/Server rigs for over 20 years so I’m sure you will be OK!
Disclaimer: All advise taken at your own risk of course! :)
Cheers
Bob