As someone who until a year ago worked in precious metal recovery, and waste disposal of photo chemistry I had numerous meetings with senior chemists and others working for UK water boards, they followed similar procedure to other EEC countries and also Canada, Australia and the rest of North America.
Effectively yes they were concerned about discharges into sewers, but while photo chemicals must be diluted they were more concerned about actual volume and the impact at treatment plants, and the residual chemical levels in their treated water.
No amateur is going to produce enough effluent to make even a minute impact, certainly not when compared to the legal discharges from pro labs etc. At work we had a permit to discharge thousands of litres of Photo chemicals per day, mainly mixed Fix, Blix and Developers, the major criteria was pH, and silver level.
As an aside I'll add that my local Water board samples the fish over a very wide region, and they get put them in our local river afterwards, where they thrive despite the high but treated input of photo chemicals.
EDTA a component in Bleach fix is actually used to treat water, so desilvered Bleach fix saves them money !
Ian