Generally, you need a visa for entry into a non-EU country and a transit visa in case you have a connecting flight via a third world country. It is often a good idea to find out visa requirement by visiting each country pages on the Foreign & Commonwealth Office website. An up to date advice on immigration, visa & entry requirements may also be collected by contacting the embassy, consulate or high commission of the destination. This is important for non-British citizens including other EU nationals.
You are advised to ensure that your travel documentation is in safe and in order prior to departure. It is likely that you will be turned away at check in if you do not have the correct documentation, Additionaly, most countries will not allow you to enter on a one-way ticket without the relevant visa or documentation.
You can check your visa requirements by visiting the following websites which contain the most up to date information on visas, passport, health & travel advise for the travellers.