A visa is not required for your trip to Guatemala or Belize, however further advice should be sought from the applicable tourist authority for other nationalities and for general queries. We strongly advise you check your destinations' Tourist Board or Embassy for the most up-to-date guidelines regarding passport validity.
It is your responsibility to check your passport validity. To enter Guatemala your passport must: have a ‘date of issue’ less than 10 years before the date you arrive and have an ‘expiry date’ at least 6 months after the day you plan to leave, as well as 2 blank pages.
To enter Belize, your passport must have an ‘expiry date’ more than 3 months after the date you arrive. Some airlines may refuse boarding if the expiry date is not more than 6 months after the date you arrive.
Sources of information include; https://www.gov.uk/foreign-travel-advice.