From 9 November 2009, the British Embassy in Guatemala is not accepting applications for British Citizen passports. Persons wishing to apply for a full validity British Citizen passport, including first passport issues, should do so by sending their applications by courier to the UK Passport Service Centre in Washington D.C. This will be the new passport production centre for Guatemala, El Salvador and Honduras.  

The mailing address is:

The UK Passport Service Centre for the Americas and Caribbean
British Embassy
19 Observatory Circle, NW
Washington, DC 20008
USA

The vast majority of applications, which have the correct supporting documentation and payment fee, will be assessed within 10 working days of receipt in Washington.

You can obtain more information, guidance and copies of application forms from http://ukinusa.fco.gov.uk/passports from the http://ukinusa.fco.gov.uk website.