Hi again. If you're seriously considering doing this as a business I would emphasize to very seriously look into the environmental regulations. If we were back in the days before this was an issue, I'd say to go for it.
I have no idea what the regulatory situation is like in London; my experience is mostly from the US, with a bit near Toronto. In these places, a proposed lab would normally apply for a sewering permit, which asks for information about the proposed effluent. The forms are tailored to the industry; in photofinishing it would include questions about total volume, BOD and COD, etc., as well as silver, and possibly even iron (iron is the key element in C-41 bleach). If acceptable, a sewering permit would be issued, specifying the exact limits on things, as well as how this will be verified (you may have to take an initial sample to a certified testing lab - who knows?)
Silver is most likely the only thing that would give you problems, and in truth, if the limits are stringent, say 0.2 mg/l silver, you would have almost no chance of meeting this. (I've designed the effluent control systems in a handful of labs, and I'm pretty certain about this.) So your hopes would be that 1) they would see you as a "small-quantity generator," and don't even regulate you, 2) they would include your effluent as part of the combined effluent stream from your entire building, or whatever, or 3) they recognize that your total "contribution" is so small that they are willing to allow a high concentration. Your only other options, as far as I can see, are that you have all the waste, including wash water, hauled by a qualified waste-disposal specialist, or, simply say nothing and hope that no regulatory body notices you. But in the latter case, consider the possibility that someone finds out and wants to "make an example" out of you.
This may be just a bunch of worry over nothing, but again, I would look into these aspects before getting too deep into things. If you're already making a living primarily as a photographer it might make more sense to farm out your processing, I dunno. If you're really concerned about a courier hauling your film, you mighty consider putting it in a locked case; you can get serialized wire seals (like your power company puts on meters), then have freight insurance, maybe through the courier service. (The lab is instructed not to open the case if the seal is broken, or if the seal number doesn't match your paperwork.)
Ps, in effluent regulations it is invariably disallowed to dilute the effluent to meet standards, fyi. (This is commonly suggested by people who have presumably not actually dealt with effluent permits.) If you are wondering what is "dilution" and what is normal "washing," have a look at the Kodak Z manuals to get wash water flow rates for certain "tank configurations." A single wash tank has the highest flow requirement; demands are lower for two counter-current flow wash tanks, etc. So if things are touchy, silver-in-effluent-wise, it's to your benefit to NOT take any steps toward water conservation - one of the unfortunate side effects of concentration-based effluent regs.
Best of luck with whatever you decide to do. I'll be glad to help with any further questions, if I can.