From page 7 of the datapoint_api_reference.pdf link https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/data/datapoint_api_reference.pdf
Example request
Fetch the three-hourly forecast for Exeter.
You need to specify the res parameter in your request. Note the ? and & positions as well as there is a typo in the pdf on page 1 that has two question marks rather than a question mark ? to start the parameter list and an ampersand & to add the key parameter.
For the forecast requests the res is either res=3hourly or res=daily for the observation request the res=hourly
http://datapoint.metoffice.gov.uk/public/data/val/wxfcs/all/xml/3840?res=3hourly&key=xxxx
Hope this helps
Alan