I thought there should be simple solid state fixed frequency inverters... a mistake obviously. In theory you could build one using a rectifier and a fixed frequency DC-AC converter like it's used for photovoltaic systems or subway/streetcar auxiliary systems (mostly lighting). Don't know if that's cheaper though.
The 380/400V difference is explained easy enough. Prior to standardization most European countries had 220/380V while the UK had 240/415V. In an effort to harmonize the nominal voltage all European countries agreed on a common voltage of 230/400V roughly 15 years ago. Some manufacturers probably still use the old specs.
