John,

I was able to get the following code to work with Forms 9.2 but it appears that http://ziptasticapi.com only works with US zip codes.  You would need to find a similar API for UK postal codes.  A quick google search =https://postcodes.io/docs might do the trick.  

Zip code input class = zip

City class = city

State class = state

//Start Zip Code Lookup
$(document).ready(function () {
  $('.zip').change(function(){
    var zipCode = $('.zip input').val();
    
    var requestURL = 'http://ziptasticapi.com/' + zipCode + '?callback=?';
    $.getJSON(requestURL, null, function(data){
      console.log(data)
      
      if (data.city) $('.city input').val(data.city);
      if (data.state) $('.state input').val(data.state);
    });
  });
});
//End zip code lookup

Hope this helps!

Nate