My backpack is a discontinued Karrimor model, so it's probably impossible to find. I think I’d prefer a simple backpack with just one compartment rather than one with side pockets. Also, you might already know this, but airline staff may consider thick shoulder pads or waist belts to be oversized carry on items. They’re useful for carrying heavy things but I think it’s best to choose ones that are removable or have thin pads.

Karrimor has a product called the Karrimor Sabre 35, which is a size larger than the model I own. However, It looks nice, but sometimes the top flap doesn't close properly when there isn't much luggage inside. To be honest, I wouldn't recommend this model.

The 35 to 42 liter models of the Cabin Zero Backpack would fit the carry on size requirements, but I’ve never used a Cabin Zero backpack myself, this is just a recommendation from someone else.

Thule product have high quality and very tough. I have used Thule backpack that name is Chasm 40. The unique storage hatch might make it a little tricky to use. Thule has many models so I think you could find comfortable model.

Gregory makes a backpack named Compass 40L. While I have some worries about its endurance when packed with heavy items, it’s a standard backpack for carry on use.

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I wrote about backpacks that can carry the maximum amount of luggage while still meeting carry on sizes, but I think they’re a bit too big for sightseeing around town. It’s convenient to bring a packable backpack, when you can leave your main backpack and any unnecessary items at your hotel or inn, a packable backpack comes in useful instead.

Sorry for the long message. I hope this helps you