It depends on your plan and where you are going. Three has some competitive international roaming add ons. Some people get Round the world roaming which has a daily fee and offers pretty much all you need, depends on the country you are going to. There is also inclusive roaming in the EU on New Pay as you go and therefore no extra charges.
Make sure you check your account to see what inclusive roaming allowances you may have and if you need to buy an add on, do it before you leave the UK. Also set up a voicemail pin because you may be required to enter the pin while roaming. Voicemail is on 123 or pressing and holding down key 1
