One more option, if it's sold in the U.K., is Nature2 with non-chlorine shock (MPS). This link seems to indicate that it is indeed available in the UK (there are other sources as well -- you can do a Google search). The Nature2 uses silver (and zinc) ions which in conjunction with the non-chlorine shock provides enough disinfection to pass EPA tests, but only in hot water (i.e. for spas, not for pools). To keep the water clear you may still need to use chlorine once every week or two, but you can do that in between soaks and if necessary could dechlorinate before getting in again (and add MPS if needed).