Is the Thule Aion Spinner 55cm accepted as a carry-on by British Airways and EL AL?

Yes. Thule lists it as compatible with both airlines' cabin limits, and its 55 cm height and 13.8-inch width fit typical sizer frames. Check your fare's weight allowance, since the bag itself weighs 7.2 lb.

How much can it hold?

The main compartment holds about 9.25 gallons, split by a zippered divider and packing straps. That is enough for roughly two to four nights of clothes, with the outer compartment for shoes or laundry.

Does it have a laptop compartment?

No dedicated padded laptop sleeve. The outer compartment can hold a small laptop or tablet, but it is not designed as a tech pocket.

Is it hard-shell or soft-sided?

It is soft-sided recycled polyester. That gives some flex when squeezing into overhead bins but less crush protection than a hard case.

How does it compare with the Thule Subterra carry-on?

Both are soft spinners with dividers. The Aion uses recycled fabric and a cleaner two-half layout, while the Subterra line adds more external pockets and a dedicated laptop sleeve on some models.