Use Google's Autocomplete Service To Search (and Output) Only Country And/or City

I initialise google places api as follows:

<script
      type="text/javascript"
      src="https://maps.googleapis.com/maps/api/js?key=API_KEY&libraries=places&types=cities"
    ></script>

and in my code, I bootstrap the service as follows:

const autocomplete = new window.google.maps.places.AutocompleteService();

autocomplete.getPlacePredictions({ input }, predictions => {
 console.log(predictions);
});

autocomplete.getPlacePredictions('London');

I'd expect the query to only return a set of cities matching that query, but instead it returns a list of London based addresses.

How could I make it return only city results matching that query?

The types property cannot be added to the Maps API script itself. You need to add it to your request to AutocompleteService.getPlacePredictions as follows:

autocomplete.getPlacePredictions({ input: 'London', types : ['(cities)'] }, predictions => {
    console.log(predictions);
});

Maps JS API script (without types):

<script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?key=API_KEY&libraries=places"></script>

Output:

London, UK
London, ON, Canada
London, KY, USA
Londonderry, UK
London, OH, USA

For reference check out the resources below:
AutocompletionRequest interface reference
Autocomplete for Addresses and Search Terms

Hope this helps!