Welcome to Rootschat

I'll answer your last sentence first.
If the headstone is an official Commonwealth War Graves Commission memorial, then the CWGC take responsibility for their maintenance. Please contact them if you feel his memorial is neglected.
Cause of death should be given on the death certificate, see reply 24, the cost since gone up
https://www.gro.gov.uk/gro/content/certificates/faq.aspThe burial at High Halden may have been to do with his place of death.
I looked at the previous posters and the main ones have not been online here since 2014.
However, as long as their email addresses haven't been changed, they may still get an email notification to let them know you have posted and may come back.
Good luck with further research.
Dawn