British nationals, travelling for the purpose of tourism, do not require a visa for visits of 90 days. You may need to present evidence of a return or onward travel on arrival. If you wish to extend your stay in Colombia, then you can apply at the Immigration office. Your passport must be valid for a minimum of 6 months from the date of entry in Colombia.
You will be required to pay a departure tax at the airport, but this is often included in the air fare. Check with your airline to confirm this.