Plants seem to like detergent. The best way to dissipate the water would be to dig shallow sloping trenches and install drain tile. The more extensive your reticulation the more efficient it will be. But you are right, you kill two birds with the one stone. You have far less sewerage to manage, and lots of irrigation water.
Looking at eggcup's post, you must remember that the deeper you site your septic tank, then the even deeper you will have to site your run-off pipes. Once you get a few metres below surface level be prepared for impermeable rock strata and very compacted soil which isn't what you want unless you are prepared get in a Cat D10 and cross-rip.
btw. The final "water" that comes out of a septic tank, is clear, doesn't smell at all, and isnutrient rich. (urine is a high quality source of nitrogen/fertiliser. urinating near plants if both good for them and very eco-friendly, though my wife doesn't see my point of view). If you go to any vivero and ask them to sell you reeds, you can plant a couple of these near your soak away area - they grow like crazy and keep the area dry.
I've just googled http://shop.septictank.co.uk/ they show all sorts of septic tanks - 4800 litre/16 person ones, eg, but they neglect to explain what happens when 16 people have a shower in the morning, flush the loo, clean their teeth, etc...........the tank would be half full and that's before the daily clothes washing, dishwashing.........hold on, it's lunchtime and everyone's back from the swimming pool and wanting another shower.....you would need - from my experience - over 100m of drainage, and even then I think your output would overtake it......and then everything would back up into your house. a gray water system would solve this problem.