My understanding is that continental Europe had 220V and the UK 240V. The EU plus a few others (CENELEC) made 230 the compromise. Equipment has to handle a wider range, e.g. my water kettle says 220 - 240. That makes life easier for manufacturers and shops. With the increase in solar and wind renewables volgage varies a lot anyway, e.g. in my apartment (NL) today it's between 235 and 240 despite the rain. Plugs tend to have shapes that fit most variants that you'll still find in older apartments.