"When a runway is notified as 'liable to slippery when wet', take-offs or landings in wet conditions should only be considered when the distances available equal or exceed those appropriate to the conditions, or, where none are specified, those required for a very slippery or icy runway, as determined from information in the Aeroplane Flight Manual" is the precise answer....
'Slippery' is probably not the same as 'very slippery', so if there's no actual information available concerning the runway surface friction or braking action, safety would suggest using the icy runway factors.