Different rules apply for countries outside the EU or EEA. A UK driving licence will suffice in some countries, but many require an International Driving Permit (IDP) in others. Further, in other countries, an IDP will be recommended but not required. For example, an IDP is generally only recommended in America - except for the state of Florida where it is compulsory. The AA website contains a list of countries which require an IDP.
It is possible to obtain an IDP, for a fee, from: the Post Office, the AA or the RAC. This is just a formality; no further tests need to be taken. But the driver must be a UK resident, have passed their driving test, and be at least 18.
Advice on driving and any driving equipment necessary in foreign countries is provided by the AA. It is important to follow the rules of the road in each country - as any driving offences committed abroad will be transferred across to local records.