Welcome to England :).
The best brands for organic yoghurt are yeovalley and rachel’s both do plain greek style yoghurts and taste delicious. You will find numerous other local brands in health food stores and farm shops, but these are the main commercial ones, although marks and spencers has just released their own organic yoghurt.
Ready made kefir can be purchased in some supermarkets in chilled world food sections or from polish specialist shops. However I am dubious as to how probiotic these products are. A good brand available from some health food stores in called nourish. It is expensive and hard to come by though, your best bet would be purchasing your own grains, easily available from ebay.
As far as other food unless you have a good food supplier by you buying online from shops like goodnessdirect.co.uk is probably easiest and cheapest, heres a voucher for £10 off your first shop http://www.goodnessdirect.co.uk/cgi-local/frameset/landing/79.html.
I’m afraid I can’t help on the water front.