The documentation seems to of implemented it now. Either way, here is the code:
geocoder.geocode(
{ 'address': address, 'componentRestrictions':{'country':'GB'}},
function(results, status){
...
});
Used to restrict results to a specific area. A filter consists of one or more of: route, locality, administrativeArea, postalCode or country. Only the results that match all the filters will be returned. Filter values support the same methods of spelling correction and partial matching as other geocoding requests. See Component Filtering in the Geocoding web service for further details.