The Location parameter can be a Zip code or a Location ID. To find your Location ID, browse or search for your city from the Yahoo! Weather home page. The weather ID is in the URL for the forecast page for that city.
You can also get the location ID by entering your zip code on the home page. For example, if you search for Los Angeles on the Yahoo! Weather home page, the forecast page for that city is http://weather.yahoo.com/forecast/USCA0638.html. The Location ID is USCA0638.
I don't know if they have the info you need for UK cities.