No satisfactory answer I am afraid, so far.
Fixing the uk 18 knife attribute is easy I can help you with that. The issue is some genious (machine or person) decided to suspend one of my ASINS in top season but they are not accepting applications to the restricted products category at the moment as they are overwhelmed with applications and it's Q4 so they are very busy. Ridiculous state of affairs.
Anyway, to solve your problem do this:
Catalogue>Add Products Via Upload
Download Spreadsheet
Update Product Details > Get Product Template
Click the words "product template"
Type the type of product in the search box (eg kitchen knife)
Tick the markets that you are selling that product in (EG amazon)
Click Generate Spreadsheet (then download the spreadsheet) You should now have a spreadsheet called kitchen knife
Open it up and look in the example tab. You want to use example 6 for a partial update
Go to the template tab and fill out the items required as per the Example tab
Specifically for an update to the Age Restriction - its column DH in the template tab
Save your spreadsheet and go back into where you downloaded the spreadsheet and click the "Upload your spreadsheet" button. Follow the steps and keep refreshing the page whilst you wait for the spreadsheet to "process".
Once processed it will either have some errors/further actions or it will have processed it. This will be an iterative process, eg you fix an error and another will crop up etc. Keep downloading the processing spreadsheet, open it up, see what the error was, fix the error, upload again, etc. Always use the last processing spreadsheet until there are no errors.
Leave it 4 hours to be safe then contact support tell them you have now fixed the issue, when you can see the "Age Verification Required: This product is not for sale to people..." text underneath the bullet points on your listing. Screenshot and send it to them. Good luck.