Guppies ?
Actually I am in south London so have very similar water to you (kettle makers must love us here), and I have been keeping all sorts of fish with no problem.
Depending on how big your tank is, you could water change with 2/3 RO water and 1/3 tap water (I buy RO water when necessary from the fish shop - there is a nice one in Beckenham), or you can start collecting rainwater.
If you buy the fish locally, they are probably used to the water anyways.
I have kept Cherry Barbs, Neons, RTB Sharks, Cories, Angelfish, Honey Gouramis, Bettas, Clown Loaches, Other Loaches....(ok, not just me, some of these are in my ex's tank, but he lives just round the corner).
The advantage is, that in Kent the water is not quite as full of Nitrates as it is in London, so you might be able to get away with not using RO or Rainwater. I wouldn't try keeping Discus, but apart from that everything else should be ok.