I was wondering if anyone knew any alternatives to septic tanks which
might be permitted to be placed in a garden.

See if you can install a Klargester Biodisc. You should have no problem
getting this past the Environment Agency if an installation is feasible. The
active aerobic process needs less space than some other solutions. However,
you might struggle to get approval within 15m of the house. You will also
need enough land for a soakaway, or a permit to discharge into a nearby
watercourse.

Alternatively, a combined system with nearby houses might be possible if
neighbourly goodwill is in abundance and someone can provide land for the
installation. You can get Biodisc systems that are suitable for large
numbers of houses (up to 75 people) and it might be attractive to your
neighbours if they're on a cess-pit that needs constant emptying, or have an
old style anaerobic septic tank that doesn't work in this day and age of
soaps and bleaches.

Christian.