British Nationals do not require a visa to enter Belize for a 30 days period. If you wish to stay for longer, you may apply and pay for extension in Belize. Your passport must be valid for a minimum period of 6 months from the date of entry into Belize.
Single parents or adults travelling with children under the age of 18 are required to present a notarised evidence of parental responsibility or consent from those with parental responsibility.
You will also be required to pay a departure tax, which is often included in the air fare. You should check with your airline to confirm this. If leaving by land, you must pay the departure tax in cash.