I should also have mentioned the RSA Substances of Low Activity Exemption Order which exempts from registration radioactive material whose activity does not exceed 0.4Bq/g (good for disposing of radioactive glass from hospitals - but that's another story). However from the title it would seem that the Environment Agency probably still regard this as radioactive. I think all the activity concentrations refer to both solid and liquid wastes.
One other interesting incidental source is the NAIR handbook (final page). Here it quotes maximum activity concentration limits in effluent following decontamination by flushing after a NAIR incident: 20 MBq/5000 litres for discharge to sewers, and 2 MBq/5000 litres for direct to watercourse.
Bye for now
Andy Hancock