Ryanair spokesman explained in The MailOnline Travel details of the incident:

"This very rare and regrettable stock shortage (on the last flight last Sunday evening) was caused by the failure of our handling agents in Murcia to deliver toilet rolls and milk sachets that had been ordered on the turnaround in Murcia. Our crew explained to passengers that we wished to prioritise an on time departure for London Stansted rather than wait for these items to be delivered and cause a significant ATC delay for all our customers. Our passengers were very understanding and we apologised sincerely to them for any inconvenience caused."

Incidents like this have happened before. In 2013, a 10-hour flight from San Fransico to London flight wasn't stocked with enough toilet paper, forcing flight attendants to improvise.