Re: there's a moderately-sized DC in central London that had water fire suppression ...
There's a large DC on the outskirts of London owned by a major bank that has water cooled cabinets housing the blade chassis with a few CO2 cooled cabinets (TROX equipment) scattered about as well.
(BTW: If the DC you're referring to is on the river, opposite Blackfriars then it does still have a water sprinkler system)