Trimming height of kitchen plinth

You will have to get someone in to do it. Anywhere you go that cuts timber to size will only cut the new timber they sell

If you're feeling handy and wanting to install it yourself, you could try taking it to a local joinery shop and ask them if they will trim it down to your required size. If it's only a small piece, they may do it for free... However, I'd expect a small fee.

I would suggest keeping your eyes open for a building site near you. If you ask politely, I am sure you could get someone to do it for you. I would oblige if someone asked me on site. Good luck