The cheapest we're aware of that's officially available in the UK is the Dogood Zero, which is £5000. We haven't tested this ourselves, but as a quadricycle-classed vehicle, it comes with many of the same compromises as the Citroen Ami, including top speed and safety limitations.

Regarding what might be available in China for less, it's hard to know what the quality of such vehicles might be like. Importing one to the UK would likely be difficult and probably quite costly, and we suspect getting insurance might be tricky.