Except that large swathes of England have incredibly hard water. I've admittedly not tried soaping with my tap water, but when you can literally see the minerals coming out of it, collecting as snow drifts of white-ish flakes in your kettle and as a hard hazy sheet on your taps, shower, basin, crockery etc, it's pretty obvious that it's likely to have a reaction of some sort... Maybe I should give it a go anyway, just to find out?
Most of the hobbyists that I know of in the UK, use either cheap-ish deionised water from the supermarket (which as I understand it, is pretty much the same as distilled water for practical purposes, but the minerals are removed using a different method) or alternative liquids like milks etc. I'm also aware of folks who make a lot of soap using a water distiller machine. It's not a cheap purchase, but at around £100 or so for a reasonable looking version, it's not really that much to invest if you make a fair bit of soap.