I don’t recall seeing anything less than what you’ve already seen. That size would fit in a wall unit as they’re 300mm deep.

B&Q do this if you want something to fit on the door:

https://www.diy.com/departments/goodhome-datil-2-tier-anthracite-steel-shelving/3663602633402_BQ.prd

This one is 30 cm deep. Steel frame, mdf shelves https://www.wayfair.co.uk/commercial-business-furniture/pdp/mercury-row-nero-5-shelf-shelving-unit-u001312968.html

https://www.amazon.co.uk/Galvanised-Shelving-Garage-Racking-DELIVERY/dp/B06XH1RZBG/ref=sr_1_19?dchild=1&keywords=Pantry+Shelving+Unit&qid=1598866424&sr=8-19

This is 26 cm deep and rather cute: https://www.amazon.co.uk/SONGMICS-Bathroom-Storage-Adjustable-Multifunctional/dp/B07T8FF4QH/ref=sr_1_20?dchild=1&keywords=Pantry+Shelving+Unit&qid=1598866424&sr=8-20

31 deep https://www.wayfair.co.uk/commercial-business-furniture/pdp/rebrilliant-creason-220cm-6-shelf-shelving-unit-udct1003.html

Maybe? I'd double check the depth with the company if it can't be any deeper than 30 cm, but... they say it's 30 deep on the dimensions drawing

https://www.amazon.co.uk/stores/page/3BFB443C-DD42-4153-BEE9-A47C872160D3?ingress=2&visitId=9ebe1bac-d232-4ccc-8210-d23db119b1c2&ref_=ast_bln

Better link: https://www.amazon.co.uk/Cozzine-5-Shelf-Shelving-Organisation-Bathroom/dp/B07DL13939?ref_=ast_sto_dp

They say it's 33 cm in the text. On the drawing it says 30 cm. But it looks like it's exactly what you're looking for?

Have you looked at Ikea's IVAR? It's wood, but 30 deep. I have it in my pantry.

Thank you for your sleuthing - lots of options - just have to find right height/width/depth combination. Rather like Cinderella and the Glass Slipper!

Hope the 30.5 cm deep one will work. If not, this one is 26 cm deep, but wood https://www.amazon.co.uk/Homfa-Adjustable-Shelving-Bookshelf-60x26x161cm/dp/B07THJDWF5/ref=pd_vtp_60_3/260-4416148-5294153?_encoding=UTF8&pd_rd_i=B07THJDWF5&pd_rd_r=e7b6535d-f01b-4da7-ae93-08c1e83e838a&pd_rd_w=YgBfW&pd_rd_wg=IY0bt&pf_rd_p=cc35f48f-2c75-4c09-90cc-cdda96af0087&pf_rd_r=A7E1MJ2JBPDF8Y88CZQJ&psc=1&refRID=A7E1MJ2JBPDF8Y88CZQJ

Decided to upgrade to bespoke stainless steel catering shelves. http://alcoengineering.co.uk in Torrington, Devon. They can make a bespoke depth for us in a week!

Perfect!

New shelves installed.

Looking good!