Why were London barges suitable for X-lighters?

How different were the London barges used for X-lighters from ordinary river barges?

They were large, sturdy, near-seagoing barges that handled choppy water and could be supplied from an established production base.

The Royal Navy selected a large, strongly built London barge that could operate in choppy water near the sea. Existing yards already produced them in numbers, making conversion faster than starting with an unfamiliar hull. Armour and propulsion turned that proven commercial form into the slow but useful X-lighter. The choice combined seaworthiness with an established production base.