Over 24 hours outage is this acceptable
Covered in the T&C's https://www.o2.co.uk/termsandconditions
No service is 100& guaranteed.
It depends on the fault and the issue, o2 wont give you a timescale for service to be restored, as there are lots of factors that impact restoration.
Also the t&c's dont guarantee a fault free network, in fact no mobile network does...