I think there's a reason this is cheap (by Antler standards) and that's because it's too big for a lot of airlines. Plus Antler have a lower budget range under the brand Revelation. I've been able to pick up hard cabin luggage for £12 and most recently £22.
Many airlines take 55 only and not 56. Personally I only have 55 for this reason.
55 40 20 for Ryanair. Good for Easyjet though. Heat added
I’ve seen people get away with much larger bags on Ryan Air...... then other times get charged for questionably larger than the cage......
I think Ryanair is 55 35 20. If so this would have to into the hold.