It’s a bit difficult to be too categorical without being able to actually have a look at your particular situation – so you’d really be well advised to get someone local to come and have a look to give you an opinion based on first-hand knowledge. That aside, I’d have to say, sadly, no, I don’t think you’re missing anything; what you describe sounds – at least on the face of it – like a pretty good solution to achieving what you’re trying to do.
You have, quite rightly, spotted one of the biggest problems with grey water recycling – namely that is really doesn’t do to have it hanging around for too long, so the systems for emptying holding tanks are essential. Equally, given the nature of the water, there’s no way to get around the filtration issue – at least not for the sort of applications you’re describing.
Unfortunately, this kind of grey water recycling technology is still in a rather early stage of development for domestic situations and the systems are – as you have noted – somewhat sophisticated, and expensive as a result. To put it bluntly, they really don’t seem to be appropriate for most British households – at least not yet and a number of environmentalists have expressed reservations over how really eco-friendly they are, given their embedded energy and running costs.
Getting Round The Problems
One solution may be to wait a bit, until they become better established – but if this isn’t an option, perhaps the best thing to do is to consider using your grey water in a more direct and immediate way.
Using grey water in your garden is the most obvious example – especially in times of drought and hosepipe bans. Ornamental planting areas are the best place, but grey water can be used on established vegetable plots, so long as you avoid too much direct contact between the water and the plants themselves – and wash them in clean water before eating them.
Garden irrigation has another major benefit – the equipment required is no more sophisticated than a few buckets or a hosepipe!
However, if you really are committed to pursuing the toilet flushing approach, then you really would be best off calling in some local expertise to come and give you an honest opinion based on what’s possible for your home.
Getting double use out of water is a great goal – so good luck if you do decide to go down this route!